This code is a 16-20 character alphanumeric string. Without it, the game remains locked in your library as a grayed-out shadow. This system prevents piracy, but it also means that "free codes" are highly controlled. Ubisoft does not simply generate codes for the public.
If you do manage to get a legitimate code (perhaps from a friend or a physical copy), here is how to activate it: Open the client. Click on the Lines icon (menu) in the top left corner. Select "Activate a Key." Enter your code in the Activation Code field. Ubisoft Connect Far Cry 3 Activation Code Free