I use this - and works fine
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<BODY OnLoad='window.setTimeout("document.login.submit()",500); '>
<form action="=>PATH TO ROUNDCUBE INDEX.PHP /mail2/index.php" method="post" name="login">
<input name="_action" value="login" type="hidden" />
<input name="_user" value="ENTER_USER_NAME" type="hidden" />
<input name="_pass" value="ENTER_PASSWORD" type="hidden" />
</form>
<br /><br /><br /><FONT FACE=Garamond SIZE=5>
Please wait while we transfer you... (Redirecting to ENTER_NAME)
</FONT>
</BODY>
</HTML>