Highlight the message and use the SecurityAssist->Show Region server behavior instead of the validation toolkit show if server behavior.
Then click on the rules manager and add a new rule.
Give it a Name: Validation Success
add one condition: Allow
value: <?php echo ((isset($_POST))?"1":""); ?>
leave compare to blank. This is for if you don't use a redirect on the page. If you do use a redirect, then add a parameter to the redirect like: pagename.php?success=true
and for the value of the rule use: <?php echo ((isset($_GET['success']))?"1":""); ?>
everything else would be the same.
Then click ok to get back to the original interface and you will be able to use that condition to show things on the page.