How to include windows header files in C++/WinRT App?

Question

I am working in a C++/WinRT project type. I need to use members from winuser.h. MSDN says to include windows.h which automatically includes winuser.h. When I do this the include statement for windows.h reports no errors. However, when I try to access members of windows.h or winuser.h in my code the compiler reports "identifier undefined". I find it strange that I would need to set a normal Win32 header file as an external dependency. Shouldn't Visual Studio be able to find these files all by itself? Does anyone know what's happening here? Am I referencing the members wrong in my code? I was under the belief that once a header file is included I could reference its members directly.