Don't do it. Sending a custom form to external recipients stands a good chance of delivering to non-Outlook recipients a Winmail.dat file, at best, and at worst, none of the attachments you might intend to send them.
I'd suggest strongly that you back up and think about why you want to use this particular form as the default. There might be a better way to accomplish the same goal.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers