Add a custom bookmarks MultiItem to the ToolbarControl
CreateBookmark.vb
' 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.
' 

Option Strict Off
Option Explicit On 

Imports ESRI.ArcGIS.Controls
Imports ESRI.ArcGIS.Carto
Imports ESRI.ArcGIS.ADF.BaseClasses

Friend Class CreateBookmarkCommand
    Inherits BaseCommand

    Private m_HookHelper As IHookHelper

  Public Sub New()
        MyBase.New()

        m_HookHelper = New HookHelperClass
        MyBase.m_caption = "Create..."
        MyBase.m_category = "Developer Samples"
        MyBase.m_enabled = True
        MyBase.m_message = "Creates a spatial bookmark based upon the current extent"
        MyBase.m_name = "Create..."
        MyBase.m_toolTip = "Create spatial bookmark"
  End Sub
  
    Protected Overrides Sub Finalize()
        m_HookHelper = Nothing
        MyBase.Finalize()
    End Sub

    Public Overrides Sub OnCreate(ByVal hook As Object)
        m_HookHelper.Hook = hook
    End Sub

    Public Overrides Sub OnClick()
        'Get a name for bookmark from the user
        Dim sName As String
        sName = InputBox("Bookmark Name", "Spatial Bookmark")
        If Trim(sName) = "" Then Exit Sub

        'Get the focus map
        Dim pActiveView As IActiveView
        pActiveView = m_HookHelper.FocusMap

        'Create a new bookmark
        Dim pBookmark As IAOIBookmark
        pBookmark = New AOIBookmarkClass
        'Set the location to the current extent of the focus map
        pBookmark.Location = pActiveView.Extent
        'Set the bookmark name
        pBookmark.Name = sName

        'Get the bookmark collection of the focus map
        Dim pMapBookmarks As IMapBookmarks
        pMapBookmarks = m_HookHelper.FocusMap
        'Add the bookmark to the bookmarks collection
        pMapBookmarks.AddBookmark(pBookmark)
    End Sub
End Class