Go to Exchange admin center at your Exchange address, e.g.
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).
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.
Select the address from the list.
Click on Add → and then on OK.