Function finish_at 

fn finish_at(
    sign_in: SignIn,
    client_id: &str,
    address: &str,
    stop: &AtomicBool,
    token_url: Option<&str>,
    store: &dyn Store,
) -> Result<Grant, SyncError>

finish, the code traded at token_url (Microsoft’s, or a stand-in in tests), the grant kept in store.