You'll still need to diagnose it- can't tell you which wires to tap into or where, since it may be those wires causing your issue. Tapping a button into bad wiring could cause a total meltdown.
I'm just spittin' in the wind, here; but I'd start with your starter relay.
With the ignition switch in "start", you should have power to the black/yellow wire terminal. If not, follow it back and clean it's connection at the bulkhead connector. If that doesn't get you power for "start", check your ignition switch and connections. Replace if you aren't getting power out of the switch's "start" terminal when key is turned to start position.
If you do have power at the black/yellow wire at the relay, then put the trans. in neutral, and check continuity of the small black wire at the relay to ground. Should be 0 or minimal ohms. This is for your neutral safety switch. If it's not grounding, it could be a fault in that wire or a bad/misadjusted NSS. The quick and dirty fix is simply to run a wire from that relay terminal to ground, bypassing the NSS circuit. Be aware that it will now start in any gear...
If you have power to the black/yellow wire in "start", and your NSS wire is grounding properly, then your starter relay is at fault. Replace it.
It sounds tougher than it is...