Log in to your admin account, go to Microsoft 365 admin center
at https://admin.microsoft.com/AdminPortal/Home#/homepage
and open the Active Users section.

Add a new service user, e.g. meetingroomapp@domain.com
using + Add a User. ⚠ Remember to set a password.

Select User (no administrator access).
You will be able to set permissions to calendars in the next step.