This function makes the frame window the active window.

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

HWND    hwndDeskTop;  /*  Desktop-window handle. */
HWND    hwnd;         /*  Window handle. */
BOOL    rc;           /*  Active-window-set indicator. */

rc = WinSetActiveWindow(hwndDeskTop, hwnd);