Horizon Forbidden West™ Complete Edition | ROW

Launch the Steam client and sign in to the account you want to bind the game to — the key will be permanently tied to it.

In the client's top menu, open the Games section.

Select "Activate a Product on Steam…".

Accept the agreement and enter the key you received — Horizon Forbidden West Complete Edition will appear in your library and you can start downloading.

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

Enter the key in the field on the page and confirm activation — the game will be added to your account library, and you can download it later in the Steam client.