Alejandro,

I have not tried this but it should work.
Keypress event

LPARAMETERS nKeyCode, nShiftAltCtrl
if nKeyCode = 13
  keyboard '{f4}'
endif 

UPDATE
Not sure what happens if enter is pressed to select from list. You may have to have a flag so keyboard won't happen when the list is showing.

>How do you open a combobox programmatically when user types {Enter} while in it?
>
>TIA,
>
>Alex