I\'m trying to create an Appointment with an Html Body containing a Hyperlink, like it is shown in this Example:
ewsAppointment = new EwsData.Appointment(service
The MS-OXCAL (section 2.2.1.38) standard document seems to suggest that appointments need to be specified in Rich Text Format. You might want to try writing the body in that, perhaps using a library like .NET RTF Writer.
I think the reason you're getting a weird change is because something in the chain is converting your HTML to RTF. This question has a lot more detail.
The property PR_RTF_COMPRESSED is what you need. It is MAPI property supported as extended property in EWS Managed API and Exchange Web Services .NET