You do not have to actually have to own the domain name before entering it into the AMS admin (settings page). You also don't have to specify a domain name before finishing the rest of the site setup steps, you can come back and do that step later.

We do, however, recommend registering your domain name as soon as possible to ensure that it's not taken by someone else. 

You will, however, need to have your custom registered before we submit your site to the payment processors for approval as it cannot be changed later.

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