Serving a application behind a different domain


We have a customer that needs to use a fixed url to target the c3 application let say
The c3 application is serving on or
The customer is using DNS resolution to quickly switch between application while the facading url remains the same (used by a frontend portal).
This is not to say that the C3 app is used with HTTPS.
What are the requirements to setup this kind of architecture?
The customer is saying that the c3 app needs to serve SSL certificate.