On MS Windows one can obtain a window handle by using:
WindowHandleToPlatform(winhandle).wnd
How does one obtain a MacOS Window "id":
for example a function that accepts a window id:
CGWindowListCreateImage(
CGRect screenBounds,
CGWindowListOption listOption,
CGWindowID windowID, // <------------------ here
CGWindowImageOption imageOption);
On Windows, WindowHandleToPlatform() retrieves a TWinWindowHandle for a given TFmxHandle. TWinWindowHandle contains (amongst other things) the window's HWND.
On OSX, WindowHandleToPlatform() retrieves a TMacWindowHandle, which contains the window's NSWindow, NSView, NSTrackingArea, and NSOpenGLView objects. You can get a CGWindowID from the NSWindow.windowNumber field, eg:
WindowHandleToPlatform(winhandle).Wnd.windowNumber