Hi....OK I've never worked on this but.....here's what I think. Refer to my attached chart. The small black/red/white from the throttle are the standard speed control lines to the controller. It appears then that they are using a 2 wire +24 volt signal from the controller to go to the key switch. Turning the key off would interrupt the +24v and turn off power. the +24 volt lines at the controller show that they are likely green and yellow. Depending on your controller (they sometimes vary) these 2 wires will go to red & blue or red & black connector. If you have a volt meter you should be able to measure resistance across the green/yellow at the throttle while switching the key. One way the wires will show zero ohms (closed/on) the opposite infinite ohms (open/off).
Here is the diagram for wiring THR-105K to SPD-24500B.
Thanks for the replies, I did manage to figure it out. Thanks Gene for confirming the power switch wires do carry 24V.