Create a third-level domain (a sub-domain) in DNS-zone of your custom domain and redirect it to GetCourse.
Then add the created sub-domain to the system in “Account settings” section - “Domains”.
Move a tuner to “Use own name server” and click on “Save” button.
IMPORTANT!After setup of redirection of sub-domain according to CNAME you won’t be able to set up a mailing with usage of this sub-domain. You have to use second-level domain in this case.