i do all the field validation stuff in a macro, displaying msgboxes if a some field is empty and such. its all being run the moment you click the save button.
Add the following to the On Click Event of the button:
If isnull(me.comboboxname) then
Msgbox "You need to select an item from the ComboBox",vbokonly,"Validation Error"
Else
Your code to run your macro here...
End if