code: function (el) {
              var genericCloseButton;
  
              if ($(el).data('popupoptions').closebuttonmarkup) {
                  genericCloseButton = $(options.closebuttonmarkup).addClass(el.id + '_close');
              } else {
                  genericCloseButton = '<button class="popup_close ' + el.id + '_close" title="Close" aria-label="Close"><span aria-hidden="true">×</span></button>';
              }
  
              if ($(el).data('popup-initialized')){
                  $(el).append(genericCloseButton);
              }
  
          }
docs: Add-close-button method
  
  @param {object} el - popup instance DOM node