For each of the available calendar modules you need to have a ical link to a shared calendar that is accessible. usually this is the “public” link or a “private” link with a long random looking string in it, that is actually still a publicly accessible link, but not discoverable, based on a personal or company name.
this is discussed here.
https://forum.magicmirror.builders/topic/11963/adding-shared-google-calendars
The “get shareable link” button you see, probably offers a link that needs explicit authentication as you have worked out.
auth is documented here in the default calendar module help
https://docs.magicmirror.builders/modules/calendar.html#configuration-options
auth is documented here in the helpfile for MMM-CalendarExt2
https://github.com/MMM-CalendarExt2/MMM-CalendarExt2/blob/master/docs/Configuration/Calendar.md
i would try different type of auth, either plain, digest or bearer as there may be restrictions on one or more types of authentication method.
[edit] I have had more luck with the MMM-calendar-Ext2 module recently
Hope this helps