<form id="sso_form" method="POST" action="http://www.drupalsite/user/login">
  <input type="hidden" name="name" value="username">
  <input type="hidden" name="pass" value="password">
  <input type="hidden" name="form_id" value="user_login">
  <input type="hidden" name="op" value="Log in">
</form>