Input the code and hit redeem and voila, you've got a Skeleton key. You can collect it from the Mail tab in the same menu. Alternatively, you can go to the Shift website and input it there.
SHiFT codes in Borderlands 3 help you redeem everything from custom skins to Diamond keys – and there are a fair few available this month. Like its predecessors and Tiny Tina’s Wonderlands ...