If you’ve bought a digital key for a game and want to activate it, there are two ways to do this:
Method 1: on Boosteroid
Click the Activate Steam Key button. You will be redirected to the page where you can enter your key.
Method 2: in the Steam app
Open Steam, click [+] in the lower-left corner, and select Activate a Product on Steam.
Follow the on-screen instructions and enter your product key. The game will be added to your library.