Office.js web add-in throws HTTP 401 Unauthorized

One of my dev customer, created Outlook Web add-in (using Office.js/JavaScript) and tried calling EWS call (mailbox.makeEWSRequestAsync method). It throws HTTP error 401 for mailbox.makeEWSRequsetAsync() method. They noticed the error or behaviour when they tested with Outlook on the Web and Outlook Web Access for devices. The API call throws, the following HTTP error: “The…