C# issue: How do I manipulate a Textbox on one form from another form

namespace LCR_ShepherdStaffupdater_1._0
    {
        partial class formErrorWindow
        {
            /// <summary>
            /// Required designer variable.
            /// </summary>
            public System.ComponentModel.IContainer components = null;

            /// <summary>
            /// Clean up any resources being used.
            /// </summary>
            /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
            ///

            protected override void Dispose(bool disposing)
            {
                if (disposing && (components != null))
                {
                    components.Dispose();
                }
                base.Dispose(disposing);
            }

            #region Windows Form Designer generated code

            /// <summary>
            /// Required method for Designer support - do not modify
            /// the contents of this method with the code editor.
            /// </summary>
// ********* HERE IS THE FUNCTION THAT IVE ADDED BELOW. THIS WOULD WORK BUT.... *********
     public static void showError(string errorTitle, string usrMsg, string errorMsg)
    {
        formErrorWindow errorWindow = new formErrorWindow();
        errorMsgItem.Text = errorMsg;
        errorTitleItem.Text = "Error! : " + errorTitle;
        usrMsgItem.Text = usrMsg;
        errorWindow.ShowDialog();
    }
// ********* HERE IS THE FUNCTION THAT IVE ADDED ABOVE. THIS WOULD WORK BUT.... *********
// ********* I get an error: "An object reference is required for the non-static field, method, or property." *********
            public void InitializeComponent()
            {
                System.ComponentModel.ComponentResourceManager resources = new System.ComponentModel.ComponentResourceManager(typeof(formErrorWindow));
                this.usrMsgItem = new System.Windows.Forms.TextBox();
                this.errorTitleItem = new System.Windows.Forms.Label();
                this.errorMsgItem = new System.Windows.Forms.TextBox();
                this.button1 = new System.Windows.Forms.Button();
                this.panel1 = new System.Windows.Forms.Panel();
                this.label2 = new System.Windows.Forms.Label();
                this.panel1.SuspendLayout();
                this.SuspendLayout();
                //
                // usrMsgItem
                //
                this.usrMsgItem.Enabled = false;
                this.usrMsgItem.Location = new System.Drawing.Point(13, 37);
                this.usrMsgItem.Multiline = true;
                this.usrMsgItem.Name = "usrMsgItem";
                this.usrMsgItem.Size = new System.Drawing.Size(334, 81);
                this.usrMsgItem.TabIndex = 0;
                this.usrMsgItem.Text = "Undefined";
                //
                // errorTitleItem
                //
                this.errorTitleItem.AutoSize = true;
                this.errorTitleItem.Font = new System.Drawing.Font("Microsoft Sans Serif", 12F, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
                this.errorTitleItem.ForeColor = System.Drawing.Color.Red;
                this.errorTitleItem.Location = new System.Drawing.Point(12, 9);
                this.errorTitleItem.Name = "errorTitleItem";
                this.errorTitleItem.Size = new System.Drawing.Size(152, 20);
                this.errorTitleItem.TabIndex = 1;
                this.errorTitleItem.Text = "Error! : Undefined";
                //
                // errorMsgItem
                //
                this.errorMsgItem.Enabled = false;
                this.errorMsgItem.Location = new System.Drawing.Point(0, 21);
                this.errorMsgItem.Multiline = true;
                this.errorMsgItem.Name = "errorMsgItem";
                this.errorMsgItem.Size = new System.Drawing.Size(329, 101);
                this.errorMsgItem.TabIndex = 2;
                this.errorMsgItem.Text = "Undefined";
                //
                // button1
                //
                this.button1.Location = new System.Drawing.Point(272, 256);
                this.button1.Name = "button1";
                this.button1.Size = new System.Drawing.Size(75, 23);
                this.button1.TabIndex = 3;
                this.button1.Text = "Continue";
                this.button1.UseVisualStyleBackColor = true;
                this.button1.Click += new System.EventHandler(this.button1_Click);
                //
                // panel1
                //
                this.panel1.BackColor = System.Drawing.Color.FromArgb(((int)(((byte)(255)))), ((int)(((byte)(128)))), ((int)(((byte)(128)))));
                this.panel1.Controls.Add(this.label2);
                this.panel1.Controls.Add(this.errorMsgItem);
                this.panel1.Location = new System.Drawing.Point(12, 124);
                this.panel1.Name = "panel1";
                this.panel1.Size = new System.Drawing.Size(335, 126);
                this.panel1.TabIndex = 4;
                //
                // label2
                //
                this.label2.AutoSize = true;
                this.label2.Font = new System.Drawing.Font("Microsoft Sans Serif", 10F, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
                this.label2.Location = new System.Drawing.Point(68, 1);
                this.label2.Name = "label2";
                this.label2.Size = new System.Drawing.Size(189, 17);
                this.label2.TabIndex = 3;
                this.label2.Text = "Technical Error Message";
                //
                // formErrorWindow
                //
                this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
                this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
                this.BackColor = System.Drawing.Color.FromArgb(((int)(((byte)(255)))), ((int)(((byte)(192)))), ((int)(((byte)(192)))));
                this.ClientSize = new System.Drawing.Size(359, 290);
                this.Controls.Add(this.panel1);
                this.Controls.Add(this.button1);
                this.Controls.Add(this.errorTitleItem);
                this.Controls.Add(this.usrMsgItem);
                this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
                this.Icon = ((System.Drawing.Icon)(resources.GetObject("$this.Icon")));
                this.MaximizeBox = false;
                this.MinimizeBox = false;
                this.Name = "formErrorWindow";
                this.StartPosition = System.Windows.Forms.FormStartPosition.CenterParent;
                this.Text = "Error!";
                this.VisibleChanged += new System.EventHandler(this.formErrorWindow_VisibleChanged);
                this.panel1.ResumeLayout(false);
                this.panel1.PerformLayout();
                this.ResumeLayout(false);
                this.PerformLayout();

            }

            #endregion

            private System.Windows.Forms.Button button1;
            private System.Windows.Forms.Panel panel1;
            private System.Windows.Forms.Label label2;
            public System.Windows.Forms.TextBox usrMsgItem;
            public System.Windows.Forms.Label errorTitleItem;
            public System.Windows.Forms.TextBox errorMsgItem;
        }
    }

pseudo: public void updateTextBox(string new_text)

MyMessageBox messageBox = new MyMessageBox("My error message");
messageBox.Show();

public Form2(Form1 form1)
        {
            InitializeComponent();

            Owner = form1;
            _form1 = form1;
        }

_form1.theForm1ControlName

public void SetErrorText(string errorText)
{
   this.ErrorMessage.Text = errorText; // assuming the TextBox' ID is ErrorMessage.
}

formErrorWindow myErrorWindow;

formErrorWindow myErrorWindow = new formErrorWindow();

formErrorWindow myErrorWindow = new formErrorWindow();
myErrorWindow.showError("Title", "User Message Here", "Error Message Here");
myErrorWindow.ShowDialog();

public partial class Form1 : Form
    {
        private static TextBox box;

        public Form1()
        {
            InitializeComponent();
        }



        private void Form1_Load(object sender, EventArgs e)
        {

            // box
            box = new TextBox();
            box.Location = new System.Drawing.Point(87, 230);
            box.Name = "box";
            box.Size = new System.Drawing.Size(100, 20);
            box.TabIndex = 1;

            this.Controls.Add(box);

        }
    }