The way I would do it if you don't want to add another button is this way.
Run an extra live wire (not the ground) through from the coin mech, to the key pin, through the button, and to the same holder as the credit switch on the board.
Messy, but if the key is turned then each time you press the button, it will give you a credit.
Alternately, use a momentary key and miss the wire to the button completely but use the rest of the wiring setup I suggested.
I think the second option would be the better of the two.