Final answer:
To specify the default business process flow in Dynamics 365 for Customer Service, configure it in system settings under the Processes section, activate it, and assign it to the relevant security roles.
Step-by-step explanation:
The default business process flow in Dynamics 365 for Customer Service can be specified by configuring it directly in the system settings. To manage business process flows and set a default, you should navigate to the Settings area of Dynamics 365. There, you will find the Processes section where you can see all the business process flows that have been created. You can open the specific business process flow you want to set as the default and ensure that it's activated. Then, you should assign it to the appropriate security roles so that users with those roles will see the process as their default process when they create a new record that the process applies to. By managing these settings, you will tailor the system to follow the desired flow for records as they are being created and processed.