Hi, MEG4444!
Thanks for reaching out to Microsoft Community and posting your inquiry regarding the camera to use on your Surface Pro (5th Gen) when using Facebook Messenger. Thanks for taking the time to let us know about this.
When you make a call, a new window will pop up showing yourself at the lower-right corner of the window.
If the call is active and you're chatting with someone, the Settings with a gear icon at the upper-right corner will appear.
That is where you can change which camera to use.
If the call ends and you close the window, you will have to redo the setup as this is a setting that is setup by the website.
If you let the window remain open and redial the call to the same person, the last camera used will be the one you see.
If you wish to have a feature like what it does on a smartphone, please contact Facebook or go to their support page to drop a feedback/suggestion.
If this answers your question, please let our Community know by marking this post as an answer. Otherwise, don't hesitate to respond to this thread if there’s anything else that needs to be addressed.
Kind regards,
Janette_Z