Add a custom bookmarks MultiItem to the ToolbarControl
CreateBookmark.cs
// Copyright 2012 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

using System;
using System.Runtime.InteropServices;
using ESRI.ArcGIS.Controls;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.ADF.BaseClasses;

namespace MultiItemBookmarks
{
    [ComVisible(false)]
  public sealed class CreateBookmark : BaseCommand
  {
    private IHookHelper m_HookHelper = new HookHelperClass();

    public CreateBookmark()
    {
      base.m_caption = "Create...";
      base.m_category = "Developer Samples";
      base.m_enabled = true;
      base.m_message = "Creates a spatial bookmark based upon the current extent";
      base.m_name = "Create...";
      base.m_toolTip = "Create spatial bookmark";
    }
  
    public override void OnCreate(object hook)
    {
      m_HookHelper.Hook = hook;
    }
  
  
    public override void OnClick()
    {
      //Get a name for bookmark from the user
      frmBookmark frm = new frmBookmark();
      frm.ShowDialog();
      int check = frm.Check; 
      string sName = "";  
  
      //OK button pressed          
      if (check == 1)
      {
        sName = frm.Bookmark;
      }
      if (sName == "") return;
      

      //Get the focus map 
      IActiveView activeView = (IActiveView) m_HookHelper.FocusMap;

      //Create a new bookmark 
      IAOIBookmark bookmark = new AOIBookmarkClass();
      //Set the location to the current extent of the focus map
      bookmark.Location = activeView.Extent;
      //Set the bookmark name
      bookmark.Name = sName;

      //Get the bookmark collection of the focus map
      IMapBookmarks mapBookmarks = (IMapBookmarks) m_HookHelper.FocusMap;
      //Add the bookmark to the bookmarks collection
      mapBookmarks.AddBookmark(bookmark);
    }
  }
}