I don't understand what you mean by "I want it to be the forest root". The forest root domain is just that-- a domain-- and not a particular server. Perhaps you mean "I want it to be a domain controller in the forest root domain" with, perhaps, the added "I also want it to hold all the FSMO roles." In either case it is not necessary to join the computer to the domain before you run
dcpromo. You can join the domain, if you like, but you don't have to, either. So long as the machine is configured with DNS server(s) that can resolve records for the domain you can run
dcpromo straight-away after installing Windows.
adprep is only necessary if you're upgrading the schema of an existing domain to the Windows Server 2008 R2 level. If the schema needs upgraded then you'll need to run
adprep against the existing domain first.
dcpromo will fail if the schema needs upgraded (and will cause no harm in doing so).