Assuming the body of the appointment item is as you have described then you can use the Outlook Word Editor to either replace the number with a barcode or append a barcode. As written the macro below will replace the number with a QR barcode and leave the item open. If you close without saving the original item will remain. If you remove the quote from the start of the two lines near the end, the text with barcodes will be copied to the clipboard and the item closed without saving.
Code:
Sub AddBarCode()
'Graham Mayor - https://www.gmayor.com - Last updated - 02 Apr 2021
Dim olItem As AppointmentItem
Dim olInsp As Outlook.Inspector
Dim wdDoc As Object
Dim oRng As Object
Dim oPara As Object
Dim i As Integer
Dim sNum As String
On Error Resume Next
Select Case Outlook.Application.ActiveWindow.Class
Case olInspector
Set olItem = ActiveInspector.currentItem
Case olExplorer
Set olItem = Application.ActiveExplorer.Selection.Item(1)
End Select
With olItem
.Save
Set olInsp = .GetInspector
Set wdDoc = olInsp.WordEditor
Set oRng = wdDoc.Range
oRng.Text = Replace(oRng.Text, Chr(11), Chr(13))
For i = 1 To oRng.Paragraphs.Count
Set oPara = oRng.Paragraphs(i).Range
oPara.End = oPara.End - 1
If InStr(1, oPara.Text, ":") > 0 Then
oPara.moveStartuntil ":"
oPara.Start = oPara.Start + 2
sNum = oPara.Text
'optional next line puts the code after the number
'oPara.collapse 0
oRng.Fields.Add oPara, 99, Chr(34) & sNum & Chr(34) & Chr(32) & "QR" & " \t", False
End If
Next i
.Display
'oRng.Copy
'olItem.Close olDiscard
End With
lbl_Exit:
Exit Sub
End Sub