Appointment API does not support multiple / shared calendars
Currently, the API has no way to identify a calendar being used to create or edit an appointment. When a user has access to multiple or shared calendars, the Outlook appointment form presents a dropdown to select a specific calendar. This property is not exposed to the API so there is no way to determine within an App the context of the calendar being used. The API should be able to expose a item.calendar property which includes the same properties for the user's mailbox such as ewsURL and token for posting the information to EWS.
Thank you for the idea! Although we appreciate your time and effort to give us this feedback, this doesn’t align with the general strategic vision of our product – we are focused on adding more capabilities to the REST API, so this is not something on our EWS roadmap right now.