OpenLinkInBackground.as

public function openLinkInBackground(url:String):void
{
    NativeApplication.nativeApplication.addEventListener(Event.DEACTIVATE, applicationDeactivateHandler);

    navigateToURL(new URLRequest(url));
}

protected function applicationDeactivateHandler(event:Event):void
{
    NativeApplication.nativeApplication.removeEventListener(Event.DEACTIVATE, applicationDeactivateHandler);

    NativeApplication.nativeApplication.activate();
}