Код:
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml" >
<mx:Script><![CDATA[
private function onComplete():void{
myHtml.htmlLoader.window.document.getElementById("text").firstChild.value = "текст вставленный внутрь сайта";
}
]]></mx:Script>
<mx:HTML id="myHtml" location="http://ya.ru"
height="100%" width="100%" complete="onComplete()"/>
</mx:WindowedApplication>