An application sends a CB_INSERTSTRING message to insert a string into the list box of a combo box. Unlike the CB_ADDSTRING message, the CB_INSERTSTRING message does not cause a list with the CBS_SORT style to be sorted.
CB_INSERTSTRING wParam = (WPARAM) index; // item index lParam = (LPARAM) (LPCTSTR) lpsz; // address of string to insert
The return value is the index of the position at which the string was inserted. If an error occurs, the return value is CB_ERR. If there is insufficient space available to store the new string, it is CB_ERRSPACE.