It's best to add accounts to your current Login (your same email address) so that you qualify of discounts. 

1. If you're looking to do a redirect (redirecting to your current website), then login to your account and register your domain via the Domains button on the Main Menu.
2. If you're looking for the additional website to have its own Hosting (separate website builder) then login to your account and go to Manage Accounts on the Main Menu.  Then click on Create Account (to create a new account which will give you an account switcher above the Dashboard on the Main Menu). From there, once you create an account you can enter your credit card and sign up for a plan.  Once you sign up for a paid plan (usually Basic or Pro), then go to domains on the Main Menu to register your domain on your new account.