Mechabellum | ROW

Launch the Steam client and sign in to the account you want to add the game to — the key is single-use and is permanently bound to the account.

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

In the activation wizard window, click "Next", accept the user agreement with the "I Agree" button, enter the received key in the Product Code field, and confirm.

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

Enter the key in the field on the page and follow the prompts — after successful activation, Mechabellum will appear in your Steam library; all that's left is to download and launch it.