Use Application control's initialize event
<?xml version="1.0"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" initialize="initApp();"> <mx:Script> import mx.managers.ToolTipManager; private function initApp():void { ToolTipManager.enabled = true; } </mx:Script> <mx:Button label="Click Me" toolTip="Click this Button to do something"/> </mx:Application>