<mx:script>
<![CDATA[[
import flash.filesystem.FileMode;
import flash.filesystem.FileStream;
import flash.filesystem.File;
private function onClick():void{
var desktop:File=File.desktopDirectory;
var saveFile:File=desktop.resolve(fileName.text);
var fs:FileStream=new FileStream();
fs.open(saveFile,FileMode.WRITE);
fs.writeUTFBytes(inputField.text);
fs.close();
}

]]
</mx:script>
<mx:Button ......click="onClick()"/>
<mx:TextInput id="fileName"...../>
<mx:TextArea id="inputField" ......./>