Upon launch of the game, you will be prompted to activate the code via Rockstar Social Club. Upon activation the user will be forced to relaunch the game to activate.

Rockstar says that you may receive the email with the activation code later than the Social Club one. There are two errors that may occur, and this is how to fix them: ...

Then he discovers that someone already used his Rockstar Activation Code, which meant someone also had used his Steam key.

Activision has partnered with Rockstar Energy drink during the launch window of Destiny 2 to offer codes that can be redeemed for in-game goods.