event GotFocus

From the code window, you select the control from the drop down list on the left and then select the GotFocus event from the drop down list on the right. This will create the GotFocus event subroutine where you would then insert your code. Be forwarned that whatever the default event for that control, usually the click event, will get it's event sub created automatically when you select the control from the list on the left.
After you select the GotFocus event from the list on the right you should go back and delete the empty event sub for the default event. If you already have code in the controls default event then it won't create another one, it will just take you to that code. Which is great because then you don't have to go back later and delete the empty sub.