If you are simply charging a low system, you should NOT have to jump the switch to charge. It is a low pressure switch and will make contact once there is enough refrigerant in the system to go above its pressure rating.

Simply start adding refrigerant on the low side. Even if the clutch is not engaged, it will draw enough refrigerant to engage the switch, turning on the compressor and then you can continue your charging process from there.

All that said, the problem you are facing is an empty system. Since you have an empty system, that means that you have a leak. If you have not already repaired the leak, make sure you put in some UV dye so that you can find the leak if the refrigerant leaks out.

Good luck,