How do I lock the app with my fingerprint?
Turn on the App lock switch under Settings → Account → Privacy. When you switch it on, your phone asks once for your fingerprint, your face or your PIN, so you know it works. It's available on every plan, on Android and iOS.
From then on the app asks to be unlocked when it starts, and when it has been in the background for 1 minute or more. If you come back within a minute, for example after sharing something or making a call, it doesn't ask again.
The setting applies to this phone only, not to the web app or your other devices. While it's on, the recent-apps screen doesn't show what's in the app.
Tip: If the switch is greyed out, set up a screen lock in your phone's settings first.
Warning: If you can't unlock it, you can sign out from the app's lock screen. Signing out turns App lock off on this phone, so turn it on again after you sign back in.