The motor is killed or turned OFF by grounding the magneto lead. So the kill switch needs to be placed it an open position when running, and connected when pulled to stop motor
In the pic below note there are 2 (M) terminals on the switch. One side goes to ground lug, this way when the key is turned OFF it crosses the contacts between the 2 which grounds out the Black/Yellow wire.
Find the Black/Yellow wire which will be connected to one side of the key switch. Connect another wire to the same terminal to the kill switch. From the other side of the kill switch run it back to the Black wire on the ground post. Same as in pic below
