If you installed a Facebook “Send to Messenger” widget before October 5, 2017, by default Facebook made all the check boxes checked by default.
This means you inherited the “pre-checked status” by default.
Facebook has since changed its policy so that all “Send to Messenger” widgets are unchecked by default. This means that once you install the PoaCart app, your checkbox is unchecked.
Obviously, you will get a lot more conversions if this checkbox is pre-checked by default. PoaCart subscription tools solve this problem.