The Package Source URL sits in the rail on every page. Copy it, then follow these stations in DSM.
Wording follows current DSM Package Center UI. Update this page if those labels change.
On your NAS, open the main menu and launch Package Center.
Main Menu → Package Center
In Package Center, open Settings (gear).
Package Center → Settings
Open Package Sources to manage third-party feeds.
Settings → Package Sources
Select Add, paste the URL from the rail, and confirm. After that, browse and install Apps inside Package Center.
Package Sources → Add → paste URL