From Xojo Documentation

Property (As String )
aListBox.List(Index as Integer) = newStringValue
StringValue = aListBox.List(Index as Integer)

Supported for all project types and targets.

The list of items. List takes one parameter, the row index. The first row is numbered zero. Use it to get or set the contents of that cell, or, in multi-column ListBoxes, the contents of the first column.