This function sets the parent for hwnd to hwndNewParent.

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

HWND    hwnd;           /*  Window handle. */
HWND    hwndNewParent;  /*  New parent window handle. */
BOOL    fRedraw;        /*  Redraw indicator. */
BOOL    rc;             /*  Parent-changed indicator. */

rc = WinSetParent(hwnd, hwndNewParent, fRedraw);