Verified answer

With the engine running and AC on start adding and it will engage when the presure gets high enough. Don't worry it will not be close to being overfilled at that point. Don't forget lubricant.

There's a pressure switch located by the evaporator remove the plug and jump the wires in the harness that was unplugged and your good to go.

When you hook up the can to charge it, the compressor should engage shortly if it is indeed low on refrigerant.

the system has a pressure switch to turn on the compressor. no pressure in system compressor will not start. get system charged. simple!