This Battle.net CD Key requires an European IP-Address. This can be done through VPN if you’re not in Europe. Activate the game by clicking on “Account Management” and then on “Add a new game”. Enter the code there and have Destiny 2 Forsaken ready for download. Read the store’s description if you need help.
A Destiny 2 Forsaken key for the Battle.net-Client does not expire and after activation Destiny 2 Forsaken can be downloaded and played forever.