How do I re fill the gearbox with oil on my Elyseo 50cc?

I have just replaced parts in gearbox and the clutch, I have re assembled it and all works ok. But I can not work out how to put the new gearbox oil in. The only hole I can find appears to be an overflow for overfill as it comes straight back out again when I try to refill through that. Please help dont want it to seize up through no oil in gearbox. Thanks