Is that metal box connected through to your electrical panel via metal conduit? If so, the box itself might be grounded, so you wouldn’t need to actually steal the power from your shed. Of course you’d want to test any outlet with the tester you have, but you’d just wire the grounded outlet like normal but w/no ground cable, and make sure the upper/lower metal tabs are screwed tight against the box.
Is that metal box connected through to your electrical panel via metal conduit? If so, the box itself might be grounded, so you wouldn’t need to actually steal the power from your shed. Of course you’d want to test any outlet with the tester you have, but you’d just wire the grounded outlet like normal but w/no ground cable, and make sure the upper/lower metal tabs are screwed tight against the box.
Also, congrats, that’s a great achievement!