You would need to create the appointment in Outlook, save it as an .ics file, upload that file to your web server, and put a link to that file in your message.
Or just attach it to the message.

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Sue Mosher, Outlook MVP
Author of Microsoft Outlook 2007 Programming:
Jumpstart for Power Users and Administrators
http://www.outlookcode.com/article.aspx?id=54
>I want to be able to put one of those Click to Attend links in a flyer, so
> that the receiver just needs to click on it and save it to their own Outlook
> Calendar. Anybody know how to set that up? Using Outlook 2003 and Exchange
> 2003.
>
> Marge