When installing a new domain comtroller with DNS, you get this warning:
A delegation for this DNS server cannot be created because the authoritative parent zone cannot be found or it does not run Windows DNS server. If you are integrating with an existing infrastructure, you should manually create a delegation to this DNS server in the parent zone to ensure reliable name resolution from outside the domain “Domain name”. Otherwise, no action is required.
Basically if you don’t have a DNS server that you want to use, you don’t have to do anything. If you have a DNS server that you want to use, you should point this DNS server to that one.