2000 Chevy Venture Thank you for telling me where the ports

I have bout a 2000 Chevy Venture from my cousin a few months ago. I am looking for the air conditioning port (ports) to add som 34a a/c refrigerant.

I found one on the top with a black unscrewable cap and the conection fits but it stays in the green in the gauge and will not go to the the light blue.

Can you kindly tell me if this is the right port on the upper right hand corner of the engine compatment.
Or is the a port underneath the vehicle?

Thank you kindly for your help. It greatly appreciated.

I'd like to donate but I have not worked in many years due to an unfortunate injury.