Log in to your admin account.

Go to the Recipients → Resources sections.
For each room used in MeetingRoomApp, set the permission to FullAccess for service user.

Select a room and click on edit (the pencil icon).
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Go to the Mailbox delegation section and scroll down to the bottom of the window content where you can find the Full Access section.

Click on + and enter the service user’s e-mail address.
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Select the address from the list.

Click on Add → and then on OK.