Microsoft Exam MS-600 Topic 10 Question 54 Discussion

Get an access token.

Note: The Outlook REST APIs require a bearer token in the Authorization header. Typically, apps use OAuth2 flows to retrieve a token. However, add-ins can retrieve a token without implementing OAuth2 by using the new Office.context.mailbox.getCallbackTokenAsync method introduced in the Mailbox requirement set 1.5.

After your add-in has the access token, item ID, and REST API URL, it can either pass that information to a back-end service which calls the REST API, or it can call it directly using AJAX.

https://docs.microsoft.com/en-us/office/dev/add-ins/outlook/use-rest-api