[oldopelguy]

That's not the fill cap, that's the vent, and it's not supposed to come off. If yours came off, then it's broken.

The fill plug is about half-way up on the side of the transmission. You fill with a "slurp" gun or similar pressure fed oiler until it starts to run out the hole.

 

[oldopelguy]

No, that is not the fill hole, it is the vent.

 

[oldopelguy]

It would drain to the right place, but how would you know how much is in there? On these transmissions you fill until oil comes out the fill hole, if you didn't even use the fill hole it would be impossible to get the right amount in there.

It's a vent, you can't plug it or it won't vent anymore. Your best bet would be to replace the whole thing, but in the mean time you could get away with putting a small piece of hose over the bolt and bending it so the open end points down to keep things from falling in there.