add freon on 1999 Ford F-250 Super Duty

where do I add freon. I'm looking for the low side pressure port.

Asked by for the 1999 Ford F-250 Super Duty
I don't suggest doing this unless you're 100% sure the system is low on refrigerant and that you're not going to overcharge the system, otherwise you risk damaging the compressor and possibly more.
I recommend paying a shop to do this so it's done correctly.
Here are some near you