Adding Text to a text box with KeyEventArgs. C#

I can't seem to have this part of my code to work. The objective for this piece of code is to add a numbered list every time The user presses, "ENTER". Here is and example of what I mean.

0)10100[User presses the ENTER key]

1)(cursor is here)

Here is the code I have. meowbox is a multiline text box.

protected override void OnKeyDown(KeyEventArgs e)
{

    if (e.KeyCode == Keys.Enter)
    {

        meowbox.Text += i + ")";
        ++i;
    }

    base.OnKeyDown(e);
}