Injecting AJAX result into a form before submitting. This is just a sample, it's not complete.

HandleFormSubmit.js

(function() {
  $(function() {
    var dataRequested = false;
    var $form = $('#form');

    $form.on('submit', function() {
      if (!dataRequested) {
        callMethodToGetAjaxData();

        return false;
      }
    });

    function callMethodToGetAjaxData() {
      var $hiddenState = $('#state_textbox');

      $.ajax({
        url...
        data...
        success: function(data) {
          dataRequested = true;

          $hiddenState.val(data.d.state);

          $('#form').submit();
        }
      });
    }
  });
})(jQuery);