This feature is not available yet. It will be available in the upcoming updates of WhatsApp.
I have many questions that if there is no way in WhatsApp to lock one or more chats with a password.
But there was no such method available. However, finally WhatsApp itself realized and announced that they are soon going to provide this feature officially.
Yes, now you can lock one or more chats with fingerprint lock. As you can see in the screenshot provided in the post, there is a locked chats folder at the top of the chats, which contains all the locked chats.
To unlock them, you will need to use your fingerprint.
