Sharing a Nextcloud Calendar in another Nextcloud Calendar

I would like to integrate a calendar (A) from another Nextcloud instance in my Nextcloud calendar (B). Sharing the calendar (A) e.g. in Thunderbird works fine - I enter the iCal link, user and password and it works.
But when I add a remote Calendar (A) in my Nextcloud Calendar (B), I can only enter the iCal link, and get the error message that the other server denies the access - seems to be right, because it probably tries with the credentials of the Nextcloud instance (A). Where can I enter the credentials of the remote Calendar?
I am using Nextcloud 17, and Calendar 1.7.1