[etc]

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:forms="*" layout="absolute" creationComplete="init()">
<mx:Script>
	<![CDATA[
		import forms.AnotherForm;
		private function init():void {
			var myForm:AnotherForm = new AnotherForm();
			this.addChild(myForm);
		}
	]]>
</mx:Script>
<!-- <forms:MyForm width="100%" height="100%" y="100"/> -->
</mx:Application>

package forms
{
	import mx.containers.Form;
	import mx.controls.ComboBox;

	public class AnotherForm extends Form
	{
		public function AnotherForm()
		{
			super();
			var cmb:ComboBox = new ComboBox();
			this.addChild(cmb);
		}
		
	}
}