property selectionIndex

This property sets and gets the index of the currently selected row.

var list = scene.getBoxById("myList");
if (list)
{
    var sel = list.selection;
    if (sel)
    {
        if (sel.selectionIndex == 1)
        {
            // process based on selection
        }
        else if (sel.selectionIndex == 4)
        {
            // process based on selection
        }
    }
}