Need to replace auto box sump gasket.(which looks simple enough)

But

How does one refill the gearbox (and check the level while doing so)?

There does not appear to be a dipstick/filler neck! I am guessing that there is a filler plug on the side of the box somewhere.

Where is the filler plug?

Do I fill the box to "overflowing" with the engine running or with the engine off? Or do I simply fill with a specific quantity of oil?