The following code illustrates adding the default size function to the system menu of a secondary window.

#define INCL_SW
#include <os2me.h>

HWND hwndFrame;

hwndFrame = WinLoadSecondaryWindow (HWND_DESKTOP, /* Parent window   */
                                    HWND_DESKTOP, /* Owner window    */
                                    MyDlgProc,    /* Dialog procedure*/
                                    NULL,         /* Module handle   */
                                    ID_DIALOG,    /* Resource ID     */
                                    NULL);        /* Create parameter*/

WinInsertDefaultSize (hwndFrame, "~Default size"); /* Insert menu item */
WinDefaultSize (hwndFrame);      /* Set window to its default size */