<form name="f" action="http://my_url_to_roundcube/?_task=mail" method="post">
Web Mail User Name: <input name="_user">
Password: <input name="_pass" type="password">
<input name="_action" value="login" type="hidden" />
<input name="_host" value="imap_host_name (e.g. localhost)" type="hidden">
<input type="submit">
</form>