Flex text box input drop-down prompt

<?xml version="1.0" encoding="utf-8"?>
<mx:FormItem xmlns:mx="/2006/mxml" label="" required="true" width="305" fontWeight="bold" height="28" horizontalAlign="right" textAlign="center" fontStyle="normal">

        <mx:Script>
        <!--[CDATA[            
            import ;
            import ;
            import ;
            import ;        

            //public var _win:PortWindow=new PortWindow();
            [Bindable]
            private var PortName:Text=new Text();

            private function DropPorts_KeyDown(event:KeyboardEvent):void
                {                    
                    if(==false)return;
                    ();
                    if(==)
                    {    
                        =;
                        if(<2)return;
                        ();
                        ="your url here";
                        (,HttpPort_result)    
                        ();
                    }
                    else
                    {    
                    }
                }
            private function HttpPort_result(event:ResultEvent):void
            {
                ();
            }    
            ]]-->
        </mx:Script>

        <mx:HTTPService method="GET" />
        <mx:ComboBox keyDown="DropPorts_KeyDown(event);"
             labelField="Name"
             dataProvider="{}" width="170" editable="true" textAlign="left"></mx:ComboBox>
        <mx:Text visible="false" height="0" width="0" text="{}" />        

</mx:FormItem>