...but I'd like to override to display a pop-up to tell the user they need to press the add/new button or the edit button first.

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Make your own TextBox Class and use it for your forms.
In the KeyPress Event of the class have a code like the following:

LPARAMETERS nKeyCode, nShiftAltCtrl
IF This.ReadOnly Then
	Wait window Nowait "Press Edit Button First!" && you can use MessageBox() function or some other notification
EndIF

HTH