It's not really a "which wire" as much as rerouting the connections. The routing will depend upon how the pot is set up. Basically, what you need to do is interrupt, or disconnect, the white line and reconnect it to either the red or black for your on/off. No way to know which way it needs to be until you test. If you pick the wrong color, the switch will be for full throttle, not off. For normal mode, you'll need to have the switch set up such that everything is connected as it is now, i.e. white line needs to stay intact and not jumped to red or black. Should be able to do this with a single pole double throw switch (SPDT). Connect the white from the board to common. Connect one switch output to the white wire on the pot. Connect the other output to either the red or black wire. Good luck and don't blow up your TX.