If you've already purchased a digital key for a video game and want to redeem the code, there are two possible ways to do that:
You can do that procedure on Boosteroid.
Just click on the "Activate Steam Key" sign and you'll be redirected to the webpage where you need to enter your digital key
Do that procedure through the Steam game client
- Run the Steam application and click on the [+] in the bottom left and choose 'Activate a Product on Steam'.
- Follow the prompts and enter your product key when asked. The game will then be added to your game library.