[david3921]

I looked at the diagram that humanbeing posted and pictures on the internet. You stated that the plug goes to the key switch and the start button. There should be a plug (male) for the one shown in the main harness. All of these tie into the starter solenoid (relay). Find that and see what that is plugged into. These typically have two wires. Follow where the wires go for that plug. It should lead to the plug you are looking for, in the main harness or bare wires where the plug should be.

As a side note, as a safety feature, that usually one of the brakes have to be applied for the ATV to start.