Dropdown Menu Button

I want to add a dropdown menu button in ckeditor toolbar which allow user to export his file in different format (txt/docx/xml...)

Here what i start to do. But don't find the way to call for my menu.

CKEDITOR.plugins.add( 'export', {
  plugins: 'menu,menubutton',
  init: function( editor ) {
    editor.addMenuGroup( 'exportation' );
    editor.addMenuItems({
      export_text : {
        label: 'Text',
        group: 'exportation',
        //command: 'export_text',
        order: 1
      },

      export_docx : {
        label: 'Docx',
        group: 'exportation',
        //command: 'export_docx',
        order: 2
      }
    });

    editor.ui.addButton( 'export', {
      label: 'Export',
     
    });
  }
});