Private WithEvents Button As Office.CommandBarButton

Private Sub Application_Startup()
  Dim oExplorer As Outlook.Explorer

  Set oExplorer = Application.ActiveExplorer
  Set Button = CreateCommandBarButton(oExplorer.CommandBars)
End Sub

Private Sub Button_Click(ByVal Ctrl As Office.CommandBarButton, CancelDefault As Boolean)
  MsgBox "Click: " & Ctrl.Caption
End Sub

Private Function CreateCommandBarButton(oBars As Office.CommandBars) As Office.CommandBarButton
  On Error Resume Next
  Dim oMenu As Office.CommandBar
  Dim oBtn As Office.CommandBarButton
  Const BAR_NAME As String = "YourCommandBarName"
  Const CMD_NAME As String = "YourButtonName"

  Set oMenu = oBars(BAR_NAME)
  If oMenu Is Nothing Then
    Set oMenu = oBars.Add(BAR_NAME, msoBarTop, , True)
    Set oBtn = oMenu.Controls.Add(msoControlButton, , CMD_NAME, , True)
    oBtn.Caption = CMD_NAME
    oBtn.Tag = CMD_NAME

  Else
    Set oBtn = oMenu.FindControl(, , CMD_NAME)
    If oBtn Is Nothing Then
      Set oBtn = oMenu.Controls.Add(msoControlButton, , CMD_NAME, , True)
    End If
  End If

  oMenu.Visible = True
  Set CreateCommandBarButton = oBtn
End Function