property selectedBox

This property sets and gets the box object that is selected. Any existing selection is overwritten with this item.

var selBox = scene.getBoxById("myListBox");
if (selBox)
{
    var bx = selBox.getBoxAtRow(4);
    if (bx)
    {
        if (bx.getAttribute("initialSelection") == "true")
        {
            selBox.selectedBox = bx;
        }
    }
}