Login from a Seperate Page

For a variety of reasons, I'd like to create my own login page. I
have found experimentally that it is insufficient to just duplicate
the Form tag from the Round Cube login page on my own page as shown
below. It seems to be realted to cookies, as it takes me back to the
Round Cube login page and flashes an error saying my browser doesn't
accept cookies. Does anybody know if what I want to do possible?

<form action="http://host/roundcubemail/" method="port"> <input type="text" name="_user"> <input type="password" name="_pass"> <input type="hidden" name="_action" value="login"> </form>