Usually, all you need to do is download the game through Steam, then during the first launch of the game, the Ubisoft application will start, where you need to log in. After logging in, the activation process happens automatically.

Where's the problem exactly? Does your Ubisoft account not have this game?
The last thing that comes to mind is copying the key that appears when launching the game on Steam, and then activating the game directly in Ubisoft.