<form id="theForm" action="/your/login" method="post">
  <input type="hidden" id="hiddenUsername" name="username"/>
  <input type="hidden" id="hiddenPassword" name="password"/>
  <input type="submit" value="Login"/>
</form>

<script type="text/javascript" language="JavaScript">
  $("#theForm").submit(function() {
    $("#hiddenUsername").val($("#username").val());
    $("#hiddenPassword").val($("#password").val());
  });
  $("#username,#password").keypress(function(e) {
    if (e.which == 13) {
      $("#theForm").submit();
    }
  });
</script>