install it before and configure it to point to the parent domain for DNS. But if you configure a secondary DNS zone on your to be child domain before it promoting it, you can point the preferred DNS to itself before promoting to a DC. This way you don't rely on forwder. Forwarder is more appropriate for external DNS or DNS in DMZ where usually tougher to get a seocndary or stub zone locally. But in your case, creating a secondard will solve many other issue before and after and when your parent DNS is not available.