Run a wire off the switched side of the switch, the one that is on only when the switch is on, and run a wire to the push button, then on from the push button to the starter. This will operate only when the switch is on.
Another way is to run a separate wire from the hot side of the switch, through the push button. This is always hot, but does not run through the switch.
Are you using the stock switch? I do not know if the stock switch could safely carry the few extra amps for the push button. Someone here would know that. If not, wire it always hot. I have a modern aftermarket switch with the starter connector in the center in the back, run it to a selonid which is wired to the starter. As I recall, you can wire that starter to operate its own selonid.