man XmListSelectItem (3): A List function that selects an item in the list

SYNOPSIS

#include <Xm/List.h>
void XmListSelectItem(
Widget widget,
XmString item,
Boolean notify);

XmListSelectItem highlights and adds to the selected list the first item in the list that matches item.

widget: Specifies the ID of the List widget from whose list an item is selected.
item: Specifies the item to be selected in the List widget. If item appears more than once in the List, only the first occurrence is matched.
notify: Specifies a Boolean value that when TRUE invokes the selection callback for the current mode. From an application interface view, calling this function with notify True is indistinguishable from a user-initiated selection action. When notify is FALSE, no callbacks are called.

For a complete definition of List and its associated resources, see XmList(3).