url to redirect back to, and <userstring> is the user-meaningful string indicating that should
usually be their login name (or something short and meaningful to distinguish multiple identities on a single AppService if applicable).
In the example above, the user Marge happens to be logged into BeerChat as 'Fred', so this identifier is attached during authorization so that when Marge manages her Presence she can see that she appears online as Fred to BeerChat.
The user is expected to already be signed into their Firefox Account in this example, otherwise they will need to first login to Prsence before seeing the authorization page.