Difficulty: using WebDAV to send MeetingRequest for a customized Form, but the DELETE event is never

[Jet]

I made a customized Appointment form with the VBScript for the event
Item_Open(),Item_BeforeDelete().
I published this form to the Exchange Server's Organizational Forms Library.

Now I can use WebDAV to create this Outlook Form in the organizer's
calendar;
and i can use WebDAV to send the MeetingRequest to the attendee.
The customized form and VBscript is also contained in the meeting request.


Here, i meet the problem:
When the attendee declines the MeetingRequest, no DELETE event
(Item_BeforeDelete) is triggered.


But the DELETE event can be triggered if the MeetingRequest is sent by
Outlook.

What property should I add to my WebDAV XML to send such a meeting request?


Thanks in advance!

Jet