As far as I know, you have to set it up using a Preferred Server and don't think that can be changed. You can make your core server a Preferred Server if you wish to. I would give some thought on the IP ranges that you give it. If you leave it blank then that will become a Preferred Server for all of your subnets. Just something to think about when setting it up.
Create a preferred server as your core server, but use an IP range that is outside of your network range. This will create your core server as a preferred server although nothing will ever use it and the HII prompt will be happy.