From what you have indicated, this isn't going to work. You cannot add code to an e-mail message and the Item_Send code would in any case have to be run from the recipients Outlook.
As written it would run on ALL e-mail messages so that wouldn't go down well.
At the very least you would have to install the code on all the user PCs so they could return your forms and then it would need some error handling to restrict it to the required message. Maybe something like the following. Then the need to check the address should be unnecessary and in any case I don't think it possible to change the recipient address at this point.
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim i As Integer
Dim bProp As Boolean
With Item
If TypeName(Item) = "MailItem" Then
If .UserProperties.Count > 0 Then
For i = 1 To .UserProperties.Count
If .UserProperties.Item(i).Name = "A1" Then
bProp = True
Exit For
End If
Next i
Else
GoTo lbl_Exit
End If
If bProp = True Then
.Body = .UserProperties("A1").Value & .Body
End If
End If
End With
lbl_Exit:
Exit Sub
End Sub