The objective is to grant FreeBusy the OAuth permissions to access calendars for users in your Office 365 domain. This consent could be given by individual users (if domain policy is to allow them to give consent) or by an admin on behalf of the users. This task covers the latter option. 

Task needs to be performed by someone who is an admin for he Office 365 tenant. Use this link to initiate a sign in request with admin consent:

Sign In For Admin Consent

On that page, use this button to sign in with your tenant admin credential:

And grant the requested permissions:

Permissions "Access your mailboxes" refers to authorizing the use of EWS (Exchange Web Services protocol) in addition to the Outlook REST API to read and write calendar data in a mailbox . "Mailbox" here does not refer to emails. FreeBusy does not use or access emails in a user's mailbox.

Note: It’s benign if you go through the process multiple times or if multiple admins do it. 

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