If a PHP domain plugin defines some mandatory parameters that must be provided during a domain purchase, it is not possible to place an order using the "Register New Domain" wizard of CCPv1. When one fills al the required parameters and tries to place an order, an error message is show indicating that one of the required parameter was not provided:
Field <...> cannot be empty
This behaviour was acknowledged as incorrect and submitted to the development team as an internal request PBA-85321. Contact the Technical Account Manager or Pooled Technical Associate Team at email@example.com to clarify the status of this request.
As an alternative, it is possible to use the "Buy More Services" wizard of CCPv1 or place an order from a Provider/Reseller Control Panel. Also, CCPv2 can be used to purchase a subscription.