How to assign default class for links

    CKEDITOR.on( 'dialogDefinition', function( ev )
            {
              // Take the dialog name and its definition from the event data.
              var dialogName = ev.data.name;
              var dialogDefinition = ev.data.definition;

              // Check if the definition is from the dialog window you are interested in (the "Link" dialog window).
              if ( dialogName == 'link' )
              {
                // Get a reference to the "Link Advanced" tab.
                var advTab = dialogDefinition.getContents( 'advanced' );
                var cssField = advTab.get( 'advCSSClasses' );
                cssField['default'] = 'cssClass';                
                
              }
            });