Hi everyone,
the beta version of the watch app is available at Fitbit Gallery App Manager
You also need the latest beta of Sleep as Android, and select Settings > Wearables > Fitbit.
This will start a service (you will see a notification) that needs to be running for the Fitbit to be able to connect to Sleep as Android, and to send data during sleep tracking.
Please direct any further question and report any experience at this thread: Fitbit Ionic/Versa beta integration