If you install "GoodLock" and then use their "Multistar" launcher widget, you have many more apps to choose from for the 8 you place on your folded app screen, one of them being FB Messenger!!! I hope it's O.K. to post a quote from a competing web site: (Quote from Android Authority article)
"The Samsung Galaxy Z Flip 5’s new Flex Window gives more room for widgets, but what about running full apps? That’s where Good Lock comes in. The app is designed to let you customize your phone’s lock screen, notification panel, and more. There’s also a special module you can download within the app, called Multistar.
Once you download the module, you can add a widget with up to eight apps that will run on the Flex Window. You can specify any app, but not all apps will work well squished down to this level, so it’s a matter of trial and error."