Hatchbox uses Caddy to automatically generate SSL certificates for your applications using LetsEncrypt.
You'll need to add add your domains to each application. This does two things. First, it tells Caddy which application to render when a request comes in from that domain. Second, Caddy can use that domain to request an SSL certificate from LetsEncrypt.
Caddy generates SSL certificates automatically during the HTTP request, so you don't have to do anything. It also will monitor the certificate for expiration and automatically renew them.