RE'd for you so you can enable window transparency for (UWP) XAML Islands
If you are using Windows Runtime Projectors (C#/WinRT (.NET 5), C++/WinRT, Rust/WinRT, etc..) use IWindowsPrivate.winmd.
For .NET Core 3.0 or older (including "UWP .NET") use IWindowsPrivate.cs. This is because you can't use winmd that has namespaces that starts with "Windows.".
((Window.Current as object) as IWindowPrivate).TransparentBackground = true;
Window::Current().as<IWindowPrivate>().TransparentBackground(true);