<?xml version="1.0" encoding="utf-8"?>
<s:SkinnablePopUpContainer
    xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:s="library://ns.adobe.com/flex/spark"
    width="300"
    height="200"
    backgroundAlpha="0.0"
    creationComplete="init()">

    <fx:Script>
        <![CDATA[
            import mx.core.FlexGlobals;
            [Bindable] private var application:Object = FlexGlobals.topLevelApplication;

            public function init():void
            {
                this.x = ( application.width / 2 ) - ( this.width / 2 );
                this.y = ( application.height / 2 ) - ( this.height / 2 );
            }
        ]]>
    </fx:Script>

    // Your content

</s:SkinnablePopUpContainer>