Create appointment via Exchange Web Services, set extended property only on organizer's instance

Background

In a web application I'm working on we're creating appointments in Exchange via the web service API. When creating an appointment, the user can select to have a booking confirmation sent out to an external partner (who is not one of the recipients of the appointment). Selecting this option results in a custom extended property being created on the appointment in Exchange开发者_运维问答. A third-party service monitors Exchange for appointments with this extended property, and sends a confirmation via SMS to the external partner whenever such a property is found.

This works, but there is a problem: the third-party service sends as many confirmations as there are attendees in the appointment, while we would like it to send only one. It seems like Exchange creates one appointment instance per attendee, with all the extended properties copied over to each instance (which is probably not something we can do anything about).

Question

Is it possible to indicate that certain extended properties should only be set on the organizer's instance of an appointment when creating it via the Exchange web service API? If so, how?