Currently, for security reasons, a custom domain must follow the pattern <subdomain>.<subdomain keyword>.<domain>. For example, https://customer.support.acme.com.
(ref: https://support.atlassian.com/organization-administration/docs/add-a-custom-domain/).

However, this is not ideal for customers who want to have a shorter custom domain.

It would be great to have the option to only use one subdomain.

In the meantime, the optional redirect URL can be used when creating the custom domain.