How can I add a Dismiss Button to a Webform Modal Conformation?
Here is how I got it working:
(function($) {
  Drupal.behaviors.hideModal = {
    attach: function(context, settings) {
        $('body', context).on('click', '#button-close-modal', function(e){
            $('.webform-confirmation-modal--content', context).dialog('close');
        });
    }  
  };
})(jQuery);