How to load URLs in a separate browser from within Samsung Messenger?
I was also looking for the same but literally there is no such option.
However, what Samsung's Official site suggests to do it, is:


  If you would prefer to use another browser as default from the messaging app - you can try searching the Google Play Store to download another messaging application. The Android Messages app, for example, defaults to open links in the Chrome browser. 


Source: https://www.samsung.com/au/support/mobile-devices/about-the-new-in-app-browser/