This feature is currently available by contacting our support team and will eventually be self-serve in Org Settings .
Important Note for SSO-Enabled Orgs
Note that if your Org has SSO enabled, you will need to updated your Issuer / Audience and Assertion Consumer Service URLs with your new subdomain. If your SSO certificate is tied to a domain, you will need to update your certificate and send us a copy as well.
Our support team can coordinate with your IT team to ensure this happens at the same time, or within a short window, to minimize any access issues.