You can purchase activation keys for Avatar games through various official and third-party retailers.
The activation process depends on which launcher you are using: Open the Ubisoft Connect PC client and log in. Avatar Pc Game Activation Key
Avatar: Frontiers of Pandora is also available through Ubisoft+ or PC Game Pass, which may bypass the need for a traditional one-time activation key. How to Activate Your Key You can purchase activation keys for Avatar games