This is an undocumented call in win32 based on DwmSetWindowAttribute but extended for use in UWP, there are plenty of issues with it, some of which might need to be addressed by William in the framework (e.g. transparent text, opaque label backgrounds, window borders/dropshadow) so it might never be usable in Xojo unless its designed into the framework. So many unknowns with this that I don't really know as I've not played with it enough to find out and as its not on my interest list I probably won't be doing so in the near future. Let us know how you get on if someone looks into it further?
The code was lifted from various places and massaged into a xojo version, see here for more info: