It’s important to note that just a CNAME might not be enough, because the Host header received will be the one with the subdomain of your site, but the discourse container will expect it to be from trydiscourse.org.
Furthermore the TLS certificate (that allows HTTPS) at trydiscourse.org expects that domain, not the one in your forum.
For question about how to configure a domain name for sites on our hosting, the best approach is to get in touch with the Discourse team directly by sending an email to the “need help” email address that’s shown on hosted site’s admin dashboard. You can also contact the team directly from the Discourse Meta forum by sending a PM to our team group. We’ll be happy to help.
As Lucas noted above, details for configuring a domain name to point to our hosting are given here: Configure your domain name for hosted Discourse. You will need to get the CNAME target that your site points to from our support team though.