Unfortunately this feature isn't in the current release. I know, it should be. :oops:
A workaround is to use JavaScript on your signup form, ie:
<form name=frm1 method=post action=http://site.com/mail/signup.php onsubmit="if(!frm1.user1){ alert('You must enter the required data!'); return false; } else { return true; }">
<input type=text name=user1>
<input type=submit value="Subscribe">
</form>