I found this code in another thread and it seems to work.
Code:
<form action="news.php" name="subscribeform" method="post">
<input type="hidden" name="subscribe_lng" value="{$store_language}" />
<input type="text" name="newsemail" size="19" />
<a href="javascript: document.subscribeform.submit();" js_to_href="Y">Subscribe</a>
</form>
Here is the link to the thread:
http://forum.x-cart.com/showthread.php?t=32615&highlight=newsletter