Firebase “Sign in with Google” is separate from Calendar here. This page only enables Calendar after googleOAuthClientId is set in vcmail.config.json and the webmail app is redeployed so window.VCMAIL_CONFIG.googleOAuthClientId is non-empty (check in DevTools). Use a Google Cloud OAuth 2.0 Client ID of type Web application; enable the Calendar API on that project. Under APIs & Credentials → that OAuth client → Authorized JavaScript origins, add this site’s origin (e.g. https://mail.yourdomain.com). Firebase’s “Authorized domains” list does not replace that step for the token client. You may reuse the same Web client ID Firebase uses if you add the webmail origin there.

You signed in with Google. Use Connect Google Calendar below with the same Google account so invitations can be added to the calendar you choose.

Connect once on this device to add .ics invitations from email into a calendar you select. Duplicate detection compares the invitation to events on the same day (by UID and start time). Access tokens stay in this browser only. After an upgrade, use Disconnect then connect again if your calendar list does not load (OAuth scopes may have changed).