Hi
First thing to determine is if you do have a plug or not
Usually they are shaped for a 3/8" sq drive extensionon your ratchet. Being a 3/8"square female pipeplug really
If you find it then you can add fluid until you can just feel the level with your pinky or until it's just dribbles back out. To add to this hole use an suction gun or one of those caps with hose that attaches right to the bottle. Anything with a hose helps.
Here's where mine is
I am 90-97 Ford Cougar application 8.8 IRS aftermarket axles
Hope it helps you
Tim