Outlook: Open Form via VBA-Makro and send Mail with settings of ThisOutlookSession

like in the headline I want to open a Form via a Button in the Ribbon of Outlook. This Form contains the MailBody of the selected Mail and some DropDown Fields. When you click on 'Send' there should pop up a MsgBox wich is defined in ThisOutlookSession, but it does not work. When I open the Form manually with the 'Choose Form', it works. Is this a bug, or is there any workaround? Thank you in advance and please ask if anything is not clear.

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I use VBA. Under Project1 in ThisOutlookSession I have got following Code:

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean) MsgBox ("Hallo") End Sub

The Form is stored/published under private forms library and is called from a module with the following code:

Set cstmControls = otlMAPIFolder.Items.Add("IPM.Note.Formular3")

But when I click the Send Button of the Ribbon the MsgBox does not appear.

The full Code of the Module, which calls the Form:

Option Explicit 
Public otlAppl As Outlook.Application
Public otlMAPINameSpace As NameSpace
Public otlMAPIFolder As MAPIFolder
Public otlMailItem As MailItem
Public cstmControls As MailItem
Public cstmUprop As UserProperties

Sub FormSend()
Set otlAppl = CreateObject("Outlook.Application")
Set otlMAPINameSpace = otlAppl.GetNamespace("MAPI")
Set otlMAPIFolder = otlMAPINameSpace.GetDefaultFolder(olFolderInbox)
Set otlMailItem = ActiveExplorer.Selection.Item(1)
Set cstmControls = otlMAPIFolder.Items.Add("IPM.Note.Formular3")
Set cstmUprop = cstmControls.UserProperties

'Body
    Dim strBody As String
    strBody = otlMailItem.Body

'Recipient
Dim strTo As String
    strTo = otlMailItem.SenderEmailAddress

strBody = strBody
With cstmControls
        .To = strTo
        .Body = strBody
        .Display True
    End With


End Sub