Please note that Facebook updated its broadcasting permissions on March 4th 2020. You can read more about it here. You must make sure you fit the criteria before you send a broadcast.
Broadcasts for Messenger bots
Broadcast messages can be scheduled in the future or sent straight away. You can only send broadcasts to people that have explicitly subscribed to broadcasts.
Step one – Compose your message
Before you can broadcast a message to your users, you need to first compose your message.
Step two – Visit your list of broadcasts
On the side bar there is a button that says ‘Broadcasts’
Step three – Schedule new post
Pick the message you want to send, and set the time for it to go out. You can edit a message which has been scheduled to broadcast up to an hour before it is due to be sent. All messages will be sent at the time you set. The time zone is displayed next to the date / time input. Messages will be sent at the same globally time to all subscribers. For example, if set to 11:00 am BST London, subscribers in Australia will receive the update at 20:00 (GMT+10). You can change your timezone on the broadcasts list page.
Step four – Wait for the results
After a few minutes past the scheduled time if you refresh the page, you will see how far along the platform is sending them. Facebook limits their use of the API, meaning that the messages have to be sent out in batches.