This function sets a window's message interest.

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

HWND     hwnd;        /*  Window handle. */
ULONG    ulMsgClass;  /*  Message class to have interest level set. */
LONG     lControl;    /*  Interest-identifier for the message class. */
BOOL     rc;          /*  Interest-changed indicator. */

rc = WinSetMsgInterest(hwnd, ulMsgClass, lControl);