From Xojo Documentation
Gets or sets the current sort column but doesn't do the sort. The first column is numbered zero. If no column is sorted then it returns -1. Call the Sort method to sort the ListBox based on the values of SortingColumn and ColumnSortDirectionAt.
The following code in the Pressed event of a Pushbutton asks the user to sort on a column if the ListBox is not sorted by any column: