Log in with the account that shares its calendar.
Right-click on the calendar, select "share" (yes, once more).
There's a popup window about the share properties.
And if you look closely, you'll see the URL to use.
It's something that looks like :
Code:
https://server.domain.tld/home/user@domain.tld/SharedCalendarName
If it's the default calendar, SharedCalendarName is Calendar.