To activate your Product Key on Steam, the first step is to install the Steam Client Application, which can be downloaded from this link.

Once you’ve downloaded and launched the app, follow these simple steps:

1. Sign in to your Steam account, and click on “ADD A GAME” as shown in the image below. You will see three options, so select “Activate a Product on Steam.”
   
2. When the Product Activation window appears, click on “Next.”
   
3. Agree to the Steam Subscriber Agreement by selecting “I Agree” in order to continue.
   
4. Copy the Game Key and paste it into the Product Code field, and then click “Next” to complete the activation process.