How to get A/C compressor to engage.

I have 1991 Subaru Leagacy. The a/c sytem was evacuated and is empty. I want to add refrigerant but how do I get the compressor to engage when the system has little to no refrigerant in it. Thanks for any help.
STEVE

You have to disconnect the low pressure switch, it should be right by the low pressure fill nipple. Anyway once disconnected you can short the connector to get the compressor clutch to engage.

Just add the freon. Once pressure builds the clutch will engage. Make sure your A/C in on Max and the fan is on high when you add the refrigerant.

Just realized the original post is from 2009

Ask for the manufacturers help and get a copy of the user manual for your compressor