If you'd like to require leads to give their email before they're able to start a conversation in the Messenger, you can turn this on from 
Automation > Basics.
Open the Leads tab and select "Require an email before a conversation starts".
You can choose to "Always" require an email to start a conversation, or "Only outside of office hours".
Note:
Only outside of office hours - Reduces conversation volume by around 5% on average.
Always - Reduces conversation volume by around 30% on average.
If you decide to turn on this setting, we recommend choosing Only outside of office hours. You can follow up with leads who got in touch while you were away, without reducing the number of leads your team have conversations with during office hours.
Once you've made your selection, click Save and turn on to make sure this automation is turned on.
If you choose Always or Only outside of office hours, the require email form will appear for leads who start a conversation, reply to a live message, or reply after engaging with a bot workflow. Leads who previously provided their email address won't be asked for it again.
Once turned on, your leads and logged-out users will be asked for their email address before they can send a message.
The Send icon will be greyed out until they leave an email address and write a message. Once they leave an email address and message, the Send icon will turn blue, so they can start the conversation.
If you don’t wish to require an email from leads to start a conversation, go to 
Automation > Basics and make sure the "Require an email before a conversation starts" automation is turned Off.
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