How to set the service FQDN

I’ve set up a stack with 2 services (a DB and a application).

I see in each service that there’s a FQDN field, and that the externalized port of each service is shown as a link, that takes me to that service.

How do I populate that FQDN field? Rancher labels? Which one?

Will that domain be used in the URL from the port link, instead of the IP?

I found nothing about this in the docs.