By default we provide the option to add custom domain on your cloud account. But it will work only with http protocol. In order to use SSL, please add certificate along with private key at Administration » System » Identity.
Certificate should be generated for your custom subdomain used for help center or wildcard certificate and key of your main domain.
Sometimes we require to apply your certificate, so you would need to submit request to our support team.
You can also generate SSL certificate right inside our system.