Create Facebook Page and "Facebook Page"->"Settings"->"Messenger Platform" tab and at "Whitelisted Domains" add domain name of your site with this plugin.
Enter "Application ID" and "Facebook Page ID" in plugin settings
If you need messages bulk sending - switch "Facebook live chat mode" to "Live chat and collecting subscribers" then "Save settings"
Enable HTTPS for your site - this in mandatory for collecting subscribers
Enter "Application ID" and "Application Secret" in plugin settings then "Save settings"
Go to your "Facebook Application Panel > Products > Facebook login", insert the "OAuth redirect URL" (from plugin settings page) to "Valid OAuth redirect URIs" then "Save settings"
Click "Connect to Facebook". After you return to plugin settings choose Facebook Page which will be connected to chat then "Save settings"