Calendar Invite Sync — Registration

This lets meeting invites sent to you automatically show up as pending events in your calendar, without needing to open the email first.

Same password you use in Thunderbird / your mail app.
Found in Nextcloud Calendar settings, or ask your admin.
Not your normal Nextcloud login password. Create one at: Nextcloud → profile icon (top right) → Settings → Security → "Devices & sessions" → type a name like "InviteSync" → Create new app password.