I’ve found what I think is an easier way.
If you put an Iframe in the page, you can redirect the exit of the action there and make it show up.
You can do nothing, of course. In that case, you can set the iframe display to none.
<iframe name="votar" style="display:none;"></iframe>
<form action="tip.php" method="post" target="votar">
<input type="submit" value="Skicka Tips">
<input type="hidden" name="ad_id" value="2">
</form>