Each time a page loads for a user you can identify, you'll want to call the FS.identify function to associate your own application-specific id with the active user.
FullStory doesn't let you change the identity of a user once they have been assigned a unique user
FS.identify. If you attempt to change the user id of an identified user, FullStory will
recognize the new user ID and automatically split the session into a new session.