How to activate a Game Code in Steam
In order to add games purchased through a third party to your Steam account, you’ll need the activation code from the third party retailer. You can either redeem codes on the Steam website or via the Steam desktop application.
To redeem a code in the Steam application, open Steam on your computer and from the top navigation bar, select “Games” and then “Activate a Product on Steam…” to start the product activation wizard.
To redeem a code via the Steam website instead, head to the Activate a Product on Steam page and sign in with your Steam account. Enter the product key here and go through the wizard. The website works basically the same as the Activate a Product on Steam option in the Steam client, but it can be used even if you don’t have Steam installed on your current device. You could even access this website from your phone.
On the first page of the wizard, you’ll be prompted to get your code ready, click “Next” when you’re ready.
On the next screen agree to the terms of service by clicking “I Agree”. On the next page of the activation wizard, enter your product code in the slot and click “Next”. Do note that the name given to the activation codes is rarely “Steam Key” when you’re buying from a different retailer and it may be labeled “Digital Activation Code”, “Product Code”, or some variation there of. The important part is that the retailer you purchase from notes that the key can be redeemed on Steam and that it is formatted like the examples seen below.
On the last screen, you’ll see a confirmation of the game title as well as a button that allows you to print a receipt of the game activation transaction. The only difference between what you see on the screen and what is printed out is that the printout includes your Steam username and a confirmation code in addition to the game title. Click finish when you’re done reviewing and/or printing the information on the final screen.
After you exit the activation wizard, you’ll see the game you just activated in your library, like so:
Simply click on the “Install” button, like you would any other Steam game, to download the game and get started playing it.