Microsoft sign-in app

Microsoft 365 and Outlook.com calendars need your own app registration. It is free and takes about 10 minutes.

Some school and work tenants block outside apps or require an admin to approve them. If yours does, use a published .ics link instead. It works one way, as a read-only source.

Register the app

Go to entra.microsoft.com and sign in.
Open App registrations and click New registration.
Give it any name, such as calsync.
For supported account types, choose Accounts in any organizational directory and personal Microsoft accounts.
Leave Redirect URI empty. Click Register.
Open Authentication. Set Allow public client flows to Yes. Save.
Open API permissions. Click Add a permission, then Microsoft Graph, then Delegated permissions. Add Calendars.ReadWrite and offline_access.
Open Overview and copy the Application (client) ID.

Registering an app needs a directory in Microsoft Entra. If your Microsoft account has none, Microsoft may ask you to create a free one first.

Give it to calsync

Paste the Application (client) ID into the calsync page. With the command line, run calsync setup microsoft.

Sign in with a device code

calsync shows a short code and tells you which Microsoft page to open. Open that page on any device, type the code, sign in and approve. With the command line:

calsync auth outlook work

Pick any label you like. The code expires after a few minutes, so do it right away.