Something

<script type="text/javascript">
$(function() {
  $(".add_url_button").click( function() {
    // Copy this part as much as necessary
    var url = $("input#url").val();
    if (url == "") {
      //$("label#name_error").show();
      $("#status_message").show();
      $("#status_message").html("The url field is empty");
      $("input#url").focus();
      return false;
    }

    // get other values
    var client = $("#client").val();
    var action = $("#action").val();
    var subaction = $("#subaction").val();
    var plugin = $("#plugin").val();

    // validate and process form here
    var dataString = 'url=' + url + '&client=' + client + '&action=' + action + '&subaction=' + subaction + '&plugin=' + plugin;

    $.ajax({
      type: "POST",
      url: "dispatcher.php",
      data: dataString,
      success: function() {
        $('#status_message').html("New transfer is added");
      }
    });
    return false;
  });
});
</script>