I see this is an old thread, but just in case you guys or anyone else needs it, the above is possible with code like below
http://mydomain.com:2095/login/
[email protected]&pass=PASSWORD&goto_uri=/3rdparty/roundcube/index.php
[/PHP]
The goto_uri variable doesn't have to be put in, I just prefer roundcube to the horde layout or squirrel layout. Enjoy!