From Xojo Documentation

Property (As Integer )
aListBox.SortedColumn = newIntegerValue
IntegerValue = aListBox.SortedColumn

Supported for all project types and targets.

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 SortedColumn and ColumnSortDirection.