For Beta testers who signed up via Google PlayStore, once you install the latest version 2.19.221, navigate to the Settings > Privacy and you will find the Fingerprint lock option.
Due to its side-server switch, this feature might not work with your first trial but users are advised not to panic, also backing up all chats is highly recommended as uninstalling the app and reinstalling might be required for some devices.
Once the option is available, setting up the fingerprint lock is similar to the same setup found in our android devices. WhatsApp would communicate to your device and permission access would be required for WhatsApp to save your biometrics (Fingerprint Data) on your phone to enable it to work with your device.
Increasing privacy just got better as options to hide notification contents displaying on your lock screen has been added.
To get the latest WhatsApp with fingerprint feature, Beta testers can head directly to the Google PlayStore or alternatively download it from APK MIRROR