Random opening on gearbox - (dip stick?)

Hello! I've been working on a bike I bought. Ive almost got it running but when I went to turn it over and fire it up oil shot out like a gizor out of a nozzle on top of the gear box. (See photos below)
Can anyone tell me what that is? Or how to plug it up?
Thanks!

That's the crankcase breather DO NOT PLUG IT UP! If oil shoots out of it when you start the engine the engine oil probably is overfilled. Put the bike on the centerstand and look at the little round glass on the right side of the engine near the bottom. The oil level, with the engine shut off, should be between the upper and lower marks on the engine case adjacent to the site glass. Ed

The stock setup has tubing connecting the crankcase breather to the airbox.
Without tubing water can get into the crankcase & contaminate the oil.
Some guys attach a breather filter, while others run a section of tubing behind the engine, forward of the swingarm --- make sure it's away from the mufflers & chain; excess oil could possibly get on the rear wheel.