In order to activate your Altergift, you will need to follow these steps:

1. Claim the purchased product on your Kinguin account. 
2. You will receive a gift link to a website. 
3. Open that link in your web browser.
4. Depending on the prompt please provide your Steam profile link or log in via Steam.
5. An automated bot system will add your Steam account to your friend list.
6. After you accept the friend request from the bot, it will send you your purchased game as a gift.

Here you can check how to activate Altergift: