Hi @TomM
There’s not really a preferred way. If you want to transfer the domain registration over to us so we manage it for you, you’d select “Transfer the domain from another registrar” and follow the steps in my previous post in this thread.
If you want to keep the registration with your current provider (Chillidog/Exact), then you can just select “Use the existing domain and update name servers” on the order form and then point the domain’s nameservers over to ours afterwards.
Whichever is easier for you. Naturally a lot of customers move their domain registrations over so that we can manage everything for them so they don’t have to bother with it. 
If you need any help with it let me know.