If you purchased Fences 4 via Steam, you won’t need an activation key. Simply install and activate it directly through the Steam client.
Before doing so, please ensure that any previous installation of Fences 4—especially one obtained via direct download from Stardock—is properly uninstalled. Once uninstalled, reboot your system immediately to complete the cleanup.
Also, I just checked and it seems Fences 4 is no longer listed for sale on Steam. I'm not sure how Humble Bundle is still offering it as a Steam-based product. You might want to reach out to the Humble Bundle team for clarification.
Thank you!
Basj,
Stardock Community Assistant