Web Designer MX Premium (v8) included top level domain name free for 12 months, in addition to the free hosting offered to all Web Designer owners by MAGIX Online World. A voucher code for the free domain would have been included in your purchase confirmation email for Web Designer MX Premium. You can then find information on how to claim you free domain here.
Web Designer 9 Premium includes 12 months MAGIX Online 'M' hosting for free, and the M hosting package includes a top level domain. You can register for your free account and domain here. You will need the coupon code that is included in the Web Designer 9 Premium purchase confirmation email.