In order to activate the product on Steam, the installation of the Steam Client will be needed. You can download it here or there.

1. First, claim the purchased product on your Kinguin account.
2. Open the Steam Client and log in to your Steam account.
3. Click the “ADD A GAME” button and select the “Activate a product on Steam” option.
4. After the Product Activation Window appears, paste your Steam Key here.
5. Click “Confirm” and then “OK”
6. Congratulations! The key is successfully activated and should be visible in your Steam Library.

Here you can check the video tutorial of Key Steam Activation: