See Remarks at:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms646280(v=vs.85).aspx

If the F10 key is pressed, the DefWindowProc function sets an internal flag. When DefWindowProc receives the WM_KEYUP message, the function checks whether the internal flag is set and, if so, sends a WM_SYSCOMMAND message to the top-level window. The WM_SYSCOMMAND parameter of the message is set to SC_KEYMENU.

Tested solution:
WindowsWindow.c 

Line 838:

+ case WM_SYSKEYDOWN:
    case WM_KEYDOWN:

This will cause the key to be detected, also:

+ case WM_SYSKEYUP:
    case WM_KEYUP:

is needed for the opposite case.