private void TextBox1_TextChanged(object sender, EventArgs e)
  {
        int first = this.textBox1.TextLength;
        string padrao = "asdfjdkjdfColocar esse texto no TextBoxqewprqewriworuoewi";
        string textIndex = padrao.Remove(0, padrao.IndexOf(this.textBox1.Text, 0) + this.textBox1.TextLength);
        this.textBox1.Text = this.textBox1.Text.Insert(this.textBox1.Text.Length, textIndex);
        this.textBox1.Select(first, this.textBox1.Text.Length);
  }

public Form1()
{
    InitializeComponent();

    this.textBox1.AutoCompleteMode = AutoCompleteMode.Append;
    this.textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;

    AutoCompleteStringCollection stringCollection = new AutoCompleteStringCollection
    {
        "asdfjdkjdfColocar esse texto no TextBoxqewprqewriworuoewi"
    };

    textBox1.AutoCompleteCustomSource = stringCollection;
}

string typed = "";

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    string padrao = "asdfjdkjdfColocar esse texto no TextBoxqewprqewriworuoewi";

    e.Handled = true;

    // TODO:: tratamento de delete, backspace, escape etc

    typed += ((char)e.KeyChar).ToString();

    this.textBox1.Text = typed;

    int indexOf = padrao.IndexOf(typed, StringComparison.OrdinalIgnoreCase);
    if (indexOf >= 0)
    {
        this.textBox1.Text += padrao.Substring(indexOf + typed.Length);
    }

     this.textBox1.SelectionStart = typed.Length;
     this.textBox1.SelectionLength = this.textBox1.Text.Length - typed.Length;

}