This function associates a pointer-conversion procedure with a window.

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

HWND    hwnd;      /*  Window handle. */
PFN     pthunkpr;  /*  Pointer-conversion procedure identifier. */
BOOL    rc;        /*  Success indicator. */

rc = WinSetWindowThunkProc(hwnd, pthunkpr);