Adding a custom domain within DeployHQ allows you to use your domain i.e. deployments.yourdomain.co.uk, then point that to your standard account domain and access it from there. It also comes with a free SSL certificate from Let's Encrypt.
In order to set up a custom domain, you will need to be subscribed to the premium plan or above. You can then set your custom domain under "Settings > Account & Access".
You'll then need to set this domain up as a CNAME with your hosting provider, pointing to cname.deployhq.com.