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

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

PSZ     pszClassName;  /*  Window-class name. */
PFN     pfnThunkProc;  /*  Pointer-conversion procedure identifier. */
BOOL    rc;            /*  Success indicator. */

rc = WinSetClassThunkProc(pszClassName, pfnThunkProc);