First, create an application and make get it ready to publish it under a custom domain name.
Make sure the application is secure and you're using strong passwords.
In this example, we're going to use the following application:
We will assign a custom domain to the "MASTER" branch of the "Drupal 8" application.
Go to the "Cusom Domains" page of the workspace
Next enter your domain information...
COPY this CNAME string and save it someplace safe.
Configure your DNS to point to this "CNAME" - this should be done with your Domain Name Registrar (like Godaddy.)
After you click the "Save" button, you will see the following screen.
You will need to upload your own SSL Certificate for this domain.
You can upload a purchased SSL Certificate or generate a free one with LetsEncrypt.
If you use a LetsEncrypt SSL Certificate, you'll have to renew/replace/rotate the cert yourself.