A combo box question

I have searched the forum for this, but could not find the answer. Please excuse if this is a repeat…

I have a combo box, which I want to set a variable when clicked…

If it were a button, I would simply code it:
variable = “value”;

my question is:

in the data field, of the properties panel, how do I tell the combo box to set a variable to fill in a dynamic text field? I think I have tried every way imaginable, but none of them work.

Thanks in advance…