Immediate Power Off Signal
In my opinion, the best wire to control power that wants to go off immediately with the engine is the power to the headlight low beam. It comes on as soon as the bike starts and goes off immediately when you shut off. It has enough power available to supply any control circuit or relay. "As I remember" (famous last words) it is available in a few places, like on the harness going forward to the headlight. It also might be available under the seat on the left side. My Clymer manual has the correct wire color (I guess yellow) but we just got a bunch of snow and the book is out in the garage and it's after 3am. I've got it all taped up now, and didn't discover the Clymer manual with a Real Wiring Diagram until after I'd had to poke around a lot.
The wire to the accessory socket is handy, just not the proper wire.
I hope that this helps. Good luck.