This was kindly posted online by R.West at the url below:-
You can only “forward as an attachment” in Outlook Web Access if you have installed the S/MIME control.
In OWA, goto OPTIONS and then EMAIL SECURITY.
Click the link to install the S/MIME attachment.
After that, you can create a NEW message, and drag/drop an existing message from your inbox into the new message. It will appear as an attachment, not a link.
*Note:- when I did this the email never showed as an attachment in the new message window but it was there and did work.