1) Add a 'point and fix' object of scrolling control from the object spy and use its mapping name
Aliases.ABCD.WinFormsObject("PopupListBoxForm", "").WinFormsObject("PopupListBox", "").WinFormsObject("VScrollBar", "").Name
OR
2) Enable the 'Autoscrolling methods' and then try.
Aliases.ABCD.AddEditDomainColumns.AutoScrolling.Enabled;
OR
3) By pressing the down arrow keys of keyboard till the desired value option is reached.
Aliases.ABCD.AddEditDomainColumns.xtraTabControl1.tbProperties.columnProperties.txtSourceExpression.Keys("[Down][Down][Down][Down]");