Depends on the gearbox, but in general, its a level/fill plug on the end of the gearbox.
Generally, remove nearside wheel-arch liner, to access the end plate of the gearbox. Towards the bottom, there should be one bolt head that's larger than the rest, probably with a copper washer behind it. Remove that and it should be a short blanking plug and a little oil should dribble out, if the car's level. If not , put more in, till it just overflows.