To put your app on your project’s custom domain name, use the CDN service.
This supports IPv6.
If your application requires DNSSEC, you are responsible for ensuring your custom domain is DNSSEC-enabled before following these instructions.
For context, Cloud Foundry’s Routes and Domains documentation explains the overall model and terminology that cloud.gov uses.
If you need to set up an app on a domain managed by GSA TTS, you may also be interested in 18F/dns.