Use ItemAdd http://www.outlookcode.com/article.aspx?id=62 where you reference the "Completed" folder.
Combine it with code like this http://www.vbaexpress.com/forum/showthread.php?5738-Need-to-Add-a-Userdefined-Property-to-Mail-Items
SAMPLE CODE
Change it so you do not update all items in the folder just the one item that triggered ItemAdd.
Option Explicit
Sub AddAUserDefinedProperty()
Dim olApplication As Outlook.Application
Dim olNameSpace As Outlook.NameSpace
Dim olFolder As Outlook.MAPIFolder
Dim olItem As Object
Dim strDomain As String
Dim olProperty As Outlook.UserProperty
Set olApplication = New Outlook.Application
Set olNameSpace = olApplication.GetNamespace("Mapi")
Set olFolder = olNameSpace.GetDefaultFolder(olFolderJunk)
For Each olItem In olFolder.Items
strDomain = Mid(olItem.SenderEmailAddress, _
InStr(1, olItem.SenderEmailAddress, "@") + 1)
Set olProperty = olItem.UserProperties.Add("Domain", olText)
olProperty.Value = strDomain
Debug.Print olItem.SenderEmailAddress, olProperty.Value
olItem.Save
Next olItem
Set olApplication = Nothing
Set olNameSpace = Nothing
Set olFolder = Nothing
Set olProperty = Nothing
End Sub
Even more reference material here http://www.codeproject.com/Articles/427913/Using-User-Defined-Fields-in-Outlook