For Messenger to work, you need a way for your customers to opt-in to the bot. We recommend using different features based on the status of your Facebook checkbox.
The checkbox can be unchecked and pre-checked:
If you haven't used the Messenger widget before September, 21 2018 your checkbox is most likely unchecked and currently there's not a way to get it pre-checked because of a change in Facebook's policy.
Therefore we recommend you use the Discount Popup widget that will prompt your visitors to opt-in and become bot followers.
Learn more about how to set up the Discount Pop-up.
If you used Messenger widget before September, 21 2018 your checkbox might be pre-checked, in which case, you are eligible to use the add to cart checkbox to send abandoned cart messages. That will trigger a Facebook message every time someone abandons a cart without unchecking that box.