What ive done there is used what Ive been told is called latching a relay. You need to join a wire into the starter pull in wire (not cut it) and run it to no.86 on the relay with a diode in the line somewhere (so when the relay is latched it will not back feed 12v to the starter and hold it on) But the headlight wire has to be physically cut and the ingition side put to no.30 on the relay and the switch side put to no.87. You can find a wire that is only on while the bike is running thats fine, but I can only really think of something to do with the charging system that is only energised while the engine is running or something to do with the actual ignition of the bike, but Ive never really liked cutting into such equipment (this is just me and in no way do I intend what ive said to mean its wrong to anyone who is confident in doing it that way).