If you purchased the Mamba Forever Edition of NBA 2K21 on Xbox One, you're entitled to a free Standard Edition of the game on Xbox Series X.
This copy of the game is part of the Xbox code for bonus content included in the physical copy of the box. It will be required to use the same Gamertag on the Xbox Series X that you used the code with.
At this time, though, the Xbox Series X isn't available yet, so we can't provide download instructions for that version. Please check back later: We'll update this article when we have more info to share.