Go find an old tether kill switch from any old thing, snowmobile, atv, watercraft, etc.
99% of the kill switches for these things (including MX bikes) are just 2 wires. When you press your MX killswitch down it connects the two wires, grounding the spark.
The tethered do the same thing but instead of having a spring holding them up (in the "open" position), they have a spring pushing them down (normally close).
Take your killswitch out and hook the 2 wires up to the 2 wires coming out of the tether killswitch. It doesn't matter what color, etc there is no wrong way to wire it up.