If you got reservoir tanks than you gradually fill the hydro until the oil stops bubbling at the bottom of each drain hole. And you just want enough oil in each tank to coat the bottom of the tank(s). Actually the tanks usually have a fill to cold line to go by. But having the top plugs out of transaxles just make refilling a lot faster as air can escape quicker.
Just filling via the remote tank(s) is just a lot slower as you got wait for the air to bleed out. Even when you think you got them fill you must purge the air from the pumps and refill.