Happy gaming! 🎮
The short answer is: However, if the game is a "Steamworks" title (meaning it uses Steam for updates and achievements), you can absolutely activate that key.
You can usually copy the key from a retailer’s email or a text file. Just highlight the code, press Ctrl + C (Windows) or Cmd + C (Mac), then click in the Steam box and press Ctrl + V (or Cmd + V ).