Can a custom calculation script on a text box show a message, but still be editable?

Hello,

I have a PDF form with a radio button group (three choices) and a textbox.  When the user chooses the third radio button of the group, I would like a message to automatically populate in the text box.  If one of the other radio buttons is chosen, the textbox shouldn't show any value by default.

Where things are getting tricky for me is that I would like for the user to be able to edit the value of this textbox, regardless of which radio button is selected.  The message that would automatically populate when the third radio button is chosen should only be a default that can be modified by the user.

Any thoughts on how I might be able to implement such a solution would be greatly appreciated!