In Web Personalization, go to Account Settings.
On the Domain Configuration page, you'll see a list of all the primary domains associated with your account. Each section lists the primary domain first (highlighted below), followed by any subdomains. Click Add Subdomain.
Enter the subdomain URL. Click Add or Delete to manage your list of subdomains, then click OK when you're done.
Your newly added subdomain will now be listed.
If you want to add primary domains to your account, please contact Marketo Support.