1-Time Notification Widget
Facebook introduced the 24 hour rule in March of 2020. This rule says that you only have 24 hours to send messages to a user. If that user doesn't engage (reply to your dialogues) then you are no longer allowed to send them a message for free. You must then use paid ads, email or sms to get them back into your bot.
Facebook then introduced the 1-Time Notification which allows you to ask a user if they would like to be notified of a specific thing in the future. This bypasses the 24 hour restriction. You must request permission from your Facebook page to use this feature. You can find out more by clicking this link.