FB messenger bot plugin needs to be inserted into page, to display optin button for users on frontend.

In visual editor use shortcode plugin button

Or in texteditor shorcode:

[fb-msg-optin msg=„Optin for notification!“]

msg ⇒ This message displayed on fronend above optin fb button

When publishing new post (or updating older one, which was not notified by FB messenger bot) check „Send one-time publish notification“ option.

After publishing/updating, all users opted for selected main category will be notified. In case that wordpress doesn't include „main category“ function, notifications will be sent to all assigned post categories.

Notifications for each post will by sent only once.