This function calls a message-filter hook.

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

HAB      hab;       /*  Anchor-block handle. */
PQMSG    pqmsg;     /*  Message to be passed to the message-filter hook. */
ULONG    ulFilter;  /*  Filter. */
BOOL     rc;        /*  Message-filter hook return indicator. */

rc = WinCallMsgFilter(hab, pqmsg, ulFilter);