As you say you have not set up your sky Q box yet but here is how i did it for sky stream (it will be exactly the same) , on your Sky Q launch netflix & select create account or get started , when asked for an email adress , put in your SKY email adress the same one you use to log into sky , this will make a new account & sighn you in to it & when you look at the account section on the phone app it will say what plan you have & "Billed through Sky" , thats how i done it , as for linking to an existing account i cant say as my existing account is linked to another provider so i just made a new one with my Sky login details, was just easier.
