[7thsky™]

Нормальным способом никак.

так можно:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application 
	xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
	
	<mx:VBox width="150" 
		horizontalCenter="0" verticalCenter="0">
		
		<mx:TextInput id="inName" width="100%"/>
		<mx:Button label="showToolTip ()" width="100%"
			click="showToolTip ()"/>
			
	</mx:VBox>
	
	<mx:Script>
		<![CDATA[
			
			private function showToolTip ():void {
				
				inName.errorString = 'Error Tool Tip - Walla';
				inName.dispatchEvent (
					new MouseEvent (MouseEvent.MOUSE_OVER));
			}
			
		]]>
	</mx:Script>
</mx:Application>