In order to activate your Rockstar Key, please follow these steps:

Activation through the website:

1. First, claim the purchased product on your Kinguin account. 
2. Register or sign in to a Rockstar Social Club account.
3. Enter your code on the Social Club Activation page
4. Paste your game code into the designated area and press the “Activate” button.

Activation through the Rockstar Games Launcher application:

1. First, claim the purchased product on your Kinguin account. 
2. Download Rockstar Games Launcher here.
3. Sign in to your Social Club account or create a new one.
4. Go to Store and click on Redeem Code.
5. Enter the code.

Here you can check how to activate the Rockstar Key: