How to start a secret conversation on Facebook messager.

Procedure to Start a Secret Conversation on Facebook Messenger
Starting a secret conversation with someone is optional because many people want Messenger to work when they switch between devices, such as a tablet, desktop computer or phone.

A user will have to tap on a friend’s name, and just above the Audio call and Video call options, you can see the ‘Secret Conversation’ option (if the update has rolled out to you).
Once you tap on Secret Conversation, you can start an end-to-end encrypted chatting session.
Note you might not be able to start a Secret Conversation with everyone, even if you have the update because the other person might still not have the feature.
FACEBOOK M ESSENGER ROLLS OUT END-TO-END E NCRYPTION FOR S ECRET C ONVERSATIONS