Getting the One-time Notification permission

The 'One-Time Notification' lets you send back-in-stock notifications if someone has been on your waitlist for more than 24 hours. 

If you're on the "Classic" Facebook Page, you can just go directly to the “Advanced Messaging” section of your FB Page Settings like this:

If you're on the New Pages Experience, please see this short video that walks you through how to get it