Scroll down and to advanced messaging features & click request
Fill the form follwing the video below and click submit for review
Check Review Status: It will check if the page have subscription messaging permission or not. Also this will store unsubscribe and resubscribe default template to database and create default label. You need to click this button alleast once after you get subscription messaging approval for your page.
Eastimate quick send reach: It will get the quick bulk broadcast estimation on current time and will get updated result.
Unsubscribe & resubscribe template settings: You can edit the default unsubscribe & resubscribe message template. Unsubscribe template will be sent to subscriber when he/she unsubscribe and will offer them to resubscribe. Another template is welcome message on resubscribe.
Broadcasting can be trgetted by labels, you can create much label you want and can bulk assign subscribers to those labels.
Here you can see the bot subscriber lis, can export subscribes and can bulk assign subscribers to labels.
Cron job command will like something like this: