Prepare Exchange calendar to work with UnSpot
To transfer rights:
Tip: permission delegation is only needed for the classic connection scheme. The new scheme is a single Exchange connection via a service account (impersonation), with no per-calendar permissions: see Connect Microsoft Exchange via a service account.
Granting the right to the user to manage the resource
In the Exchange admin center go to recipients > resources, select the meeting room mailbox and add the user under mailbox delegation.
Note: If necessary, create a mailbox for the user and a resource room in advance.
Delegating a calendar to a user
Sign in to Outlook on the web under the service account, open Calendars > Add calendar > From directory and select the resource.
Next step: connect the calendar in UnSpot
Permissions on the provider side are only half of the job. The calendar still has to be linked to a specific meeting room in the UnSpot console: in the admin panel that is Manage → Office management → Workspaces, the Connect calendar button on the meeting room row.

The full walkthrough — choosing On-premise Microsoft in the connection window, filling in the server connection form and picking the calendar — is in Connect an Exchange calendar to a meeting room. If the calendar you need is missing from the list at the last step, the permissions from this article have not been granted yet.