Guidance Needed: Syncing Appointments Between New Outlook and Web App via Outlook Add-in

Your SOAP application's login does not authorize access to a Microsoft calendar. Graph requires a Microsoft access token with appropriate calendar permissions; hosting an add-in inside Outlook does not remove that requirement.

Separate creating the current appointment from ongoing synchronization. In appointment compose mode, Office.js can populate and save the current item with saveAsync, without making a Graph call. That does not provide background synchronization with your SOAP system.

For Graph access, consider a multitenant app registration and MSAL nested app authentication for supported work mailboxes. It can reuse the host's signed-in identity, reducing extra prompts, but consent and Conditional Access can still require interaction. Customers do not need a new external federation service just for this integration.

Put ongoing synchronization in an authorized backend: map the SOAP and Outlook event identifiers, process changes in both directions, and use Graph change notifications plus calendar-view delta queries for Outlook-side reconciliation.