App registration in Azure is needed in order to use OAuth 2.0 to connect Microsoft 365 via EWS (Exchange Web Services).
To create an application:
Log in to Azure.
Click on App registrations.
Add a Client Secret.
Alternative Permissions (Impersonation role required)
Click on Grant admin consent.
Copy IDs and the secret token to the calendar provider.
Paste the copied information into the Calendar Providers section.