Sub Test()
Dim olMsg As MailItem
On Error Resume Next
    Select Case Outlook.Application.ActiveWindow.Class
        Case olInspector
            Set olMsg = ActiveInspector.currentItem
        Case olExplorer
            Set olMsg = Application.ActiveExplorer.Selection.Item(1)
    End Select
    AddData olMsg
    Set olMsg = Nothing
End Sub

Sub AddData(olItem As MailItem)
'Graham Mayor - https://www.gmayor.com - Last updated - 07 Jul 2021
Dim olInsp As Outlook.Inspector
Dim wdDoc As Object
Dim oRng As Object
Dim sCase As String, sRef As String

    sCase = InputBox("Enter the case number")
    sRef = InputBox("Enter the reference number")

    With olItem
        .BodyFormat = olFormatHTML
        Set olInsp = .GetInspector
        Set wdDoc = olInsp.WordEditor
        Set oRng = wdDoc.Range
        oRng.collapse 1
        oRng.Text = "++++++++++++++++ Admin use only ++++++++++++++++++++" & vbCr & vbCr & _
                    "Case: " & sCase & vbCr & "Reference: " & sRef & vbCr & vbCr & _
                    "++++++++++++++++++++++++++++++++++++++++++++++++++" & vbCr & vbCr
        oRng.collapse 0
        oRng.Select
    End With
lbl_Exit:
    Set olInsp = Nothing
    Set wdDoc = Nothing
    Set oRng = Nothing
    Exit Sub
End Sub