This function returns the active window for HWND_DESKTOP, or other parent window.

#define INCL_WINWINDOWMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND    hwndParent;  /*  Parent-window handle for which the active window is required. */
HWND    hwndActive;  /*  Active-window handle. */

hwndActive = WinQueryActiveWindow(hwndParent);