An option "Parent Subscription Required" is set to "Yes" in the domain service template. When trying to purchase domain for a customer without any subscriptions in CCP v1 - there is an exception:
To purchase the Domain <name> you must specify a hosting Subscription. Unfortunately, none of your Subscriptions can be used for this purpose.
However it is possible to perform the same operation for the same customer in CCP v2.
Additionally, there is no way in CCP v2 to assign a hosting subscription to the domain one.
Parent subscriptions are not supported in the domain purchase wizard in CCP v2. There is a feature request with id #PFR-1149 ("assign domain to parent subscription in CCPv2") to implement the described functionality in CCP v2.