Is it possible to have a command button click event autofill a textbox within a userform?

I'm attempting to use a command button to autofill a text box (both within the same userform) with a value. I'd also like to add functionality to allow for toggling to remove the value the command button entered in the first time. The command button click would also disable input into the text box unless the command button is clicked again. I just happen to be stuck on the very first part of this issue.

Here's the code I was attempting to use but alas, no joy: (Command Button name is N/A and TexBox name is A1). I've attached a sample to make it easier.

Private Sub NA1_Click()
    If NA1.Value = True Then
    A1.Value = "N/A"
    End If
End Sub

Any help would be greatly appreciated. Thanks in advance!

-Orestees