Facebook Messenger
Facebook Messenger can be added to your site in 2 ways:
Enable the chat plugin on all pages
Enable the Messenger checkbox on checkout pages
Chat Plugin
The Facebook Chat Plugin can be automatically placed on all pages on your site.
Go to Settings > Add Ons
Enter your Facebook Page ID and check Enable chat plugin
Press Save & Continue
Publish
The Chat Plugin is setup at Facebook at Page Settings > Messaging
Messenger Checkbox
The Facebook Messenger Checkbox can be placed on your checkout page. This allows consumers to opt in to communication with you.
Go to Settings > Add Ons
Enter your Facebook Messenger App ID and your Facebook Page ID.
Press Save & Continue.
Edit your checkout page(s)
Add a Form checkbox to the page. Position it where you want the Messenger checkbox to be (your checkbox will be replaced by the FB box). Give it the name “messenger”.
Save the page
Exit the page builder
Publish