How to set Combo object as readonly?

var CBoxList = ["UC-0079", "UC-0080", "UC-0083", "CE-0072", "CE-0073"];
        var CBoxValue = ["U79", "U80", "U83", "C72", "C73"];
        <!--	Object ZONE	-->
        var combo = new AW.UI.Combo;
        combo.setId("cbox");
        combo.setName("cbox");
        combo.setControlText("favorites Story : ");
        combo.setControlImage("favorites");
        combo.setItemText(CBoxList);
        combo.getContent('box/text').setAttribute('readonly','readonly');// -> Not work
        //combo.setItemImage(CBoxValue);
        combo.setSize(120,20);
        combo.setItemCount(CBoxList.length);
        combo.refresh();