No, you don't set it automagically. I haven't really gone through this in detail, but there should be several ways to get this done.
You can add an attribute in the registration form. Test for the value of this attribute during authentication.
OR you add the attribute after first authentication. Check how the Enf profile "Guest MAC Caching" adds attributes to the Endpoint. Here you can for example use Source (which is the name of the registration page).