There's a metal tube that goes from the overflow hole at the top of your tank to the bottom of your tank, ending at your pictured connector and then continuing via a rubber tube to the underside of the bike. You likely have a crack in this tube. If you're only getting leakage when the tank is full the crack is at the top of the tank, probably where this tube makes a 90 degree turn. If you get it all the time the crack is at the bottom of the tube.
Two ways to fix:
1: Drain tank, remove pump and coat tank with sealer, paying special attention to "paint" this tube so sealer coats it very well.
2. Bend the rubber overflow tube 180 degrees and seal off tightly with a zip tie. You will lose your overflow drain 'feature' but it's not really that important. How often do you really overfill your bike?
You can verify that this is the problem by blowing into the overflow tube while someone looks into your gas tank. You'll see air bubbles in your gas.
Obviously you'll only see bubbles if the crack is below your fuel level so you may need a full tank of gas. If you don't have a full tank and don't see bubbles the crack is above the fuel level. In this case you shouldn't be getting any leakage through the overflow tube.