Support confirmed the only ways to set up the new "Manager Notifications" feature are to select Learners and manually enter the Manager's email address, or to configure the "manager" field in your synced provider (e.g.: Entra ID).

If you don't have Entra synced, or don't want to use the synced provider's "manager" field value for this setting, it would be nice to have a default "Manager" setting in SAT so you don't have to either rely upon the upkeep of a synced provider's directory or manually enable it for each user - that would be tough to keep up with.