Platforms like Humble Bundle or Green Man Gaming sell official Steam keys.

Most key generators hide ransomware, spyware, or crypto-miners inside the download.

To activate , you can use either the built-in game activation screen for retail/direct versions or the Steam client if you purchased a digital key. Activation Guide 1. Retail or Non-Steam Version

Older boxed copies of the game contain the activation code printed on a leaflet inside the case. Step-by-Step Activation Guide