1. Login to My.Presonus.com.
2. Click on MyProducts.
3. You should see Prime, including your Product Key, but if you activate it online I don't think it will ask you for it.
4. Download the installer.
5. Install Prime
6. Under the Studio One menu, select: Studio One Activation...
Follow the on-screen instructions. making sure you select Activate Online
NOTE: You should only need the Product Key if you try to activate offline.