XmListAddItem man page
XmListAddItem — A List function that adds an item to the list
#include <Xm/List.h> void XmListAddItem( Widget widget, XmString item, int position);
XmListAddItem adds an item to the list at the given position. When the item is inserted into the list, it is compared with the current XmNselectedItems list. If the new item matches an item on the selected list, it appears selected.
Specifies the ID of the List to which an item is added.
Specifies the item to be added to the list.
Specifies the position of the new item in the list. A value of 1 makes the new item the first item in the list; a value of 2 makes it the second item; and so on. A value of 0 (zero) makes the new item the last item in the list.
For a complete definition of List and its associated resources, see XmList(3).