<form action="url_to_roundcube-login" method="post" name="rcLogin" id="rcLogin">
<input name="_action" type="hidden" value="login">
E-Mail: <input name="_user" type="text"><br />
Password: <input name="_pass" type="password"><br />
<center><input name="button" type="Submit" value="Login"></center>
</form>
this one works. thanx