textbox.ps1

Add-Type -AssemblyName System.Windows.Forms

Function Button1_Click() {
    If ([Windows.Forms.MessageBox]::Show($TextBox1.Text + "`n`nAre you sure?", "Question", [Windows.Forms.MessageBoxButtons]::YesNo) -eq [Windows.Forms.DialogResult]::Yes) {
        [Windows.Forms.MessageBox]::Show("Action!")
    }
}

$Form1 = New-Object System.Windows.Forms.Form
$Form1.Text = "ask you"
$Form1.Width = 275
$Form1.Height = 330
$Form1.FormBorderStyle = [Windows.Forms.FormBorderStyle]::Fixed3D

$Button1 = New-Object System.Windows.Forms.Button
$Button1.Text = "go"
$Button1.Top = 260
$Button1.Left = 90
$Button1.Add_Click({Button1_Click})

$TextBox1 = New-Object System.Windows.Forms.TextBox
$TextBox1.Multiline = $True
$TextBox1.Left = 10
$TextBox1.Top = 10
$TextBox1.Width = 240
$TextBox1.Height = 240
$TextBox1.ScrollBars = [System.Windows.Forms.ScrollBars]::Both

$Form1.Controls.Add($Button1)
$Form1.Controls.Add($TextBox1)
$Form1.ShowDialog()