Warhammer 40,000: Boltgun | GL

Launch the Steam client and sign in to the account you want to add the game to.

In the client's top menu, open the "Games" section and select "Activate a Product on Steam…".

Complete the activation wizard: click "Next" and accept the Steam Subscriber Agreement with the "I Agree" button.

Enter the key in the "Product Code" field and click "Next" — once confirmed, the game or expansion will appear in your Steam library.

Via browser: open store.steampowered.com/account/registerkey and sign in to the same Steam account.

Paste the key into the field on the page, confirm that you agree to the terms, and click the continue button — the game will be linked to your account and become available in the Steam client.