How to turn on fingerprint lock in WhatsApp 

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WhatsApp for Android
Open WhatsApp.
Tap Settings.
Tap Account.
Tap Privacy.
Go to Fingerprint Lock.
Switch on Unlock with fingerprint, and then confirm your fingerprint.
WhatsApp for iOS
Open WhatsApp.
Tap Settings.
Tap Account.
Tap Privacy.
Go to Screen Lock.
Turn on Touch ID or Face ID.
You can then tap to select the amount of time before Touch ID or Face ID is prompted.