How to Force Compressor Clutch to Engage

1990 pickup 22RE. Will replace receiver/dryer. I’ve read that after pulling vacuum on system that you are supposed to force engage the compressor clutch before trying to add R134a to an empty system. However, I do not see how it is done on this model. There is no relay in the engine to jump with a paper clip. Can someone tell me exactly how to force engage this compressor clutch in order to start adding R134a?

 

If you have pulled a vacuum and the system is leak free there is no reason to force the compressor into running. The vacuum on the system will pull enough refrigerant in to run the system. In most cases you can pull the entire recommended charge in without running the system.
If you have lost the vacuum before you charge it, you have defeated the purpose of vaccing it down in the first place.