Visit the Fantasy Grounds website and go to Store > Order History. All of your license keys or subscription based licensed keys will be shown there.
Open Steam on your computer and go to your Steam Library.
Right-click on Fantasy Grounds Unity and select Manage, then CD Keys
Copy the Product Code to the clipboard and paste it into your Settings in the screens shown below. It will automatically validate and should give you the Valid [License Type] License message. This message will change depending on whether a Standard or Ultimate license was entered.