The "Regenerate Code" button, unfortunately, won't save any custom changes to the code in the text box.
Does your complete form look something like this?
<form method=post action=http://site.com/mail/signup.php>
<input type=hidden name=overwritedupes value=1>
<input type=hidden name=list value=1>
Email: <input type=text name=email><br>
<input type=submit name=sup value="Subscribe Me!">
</form>
Regards