While creating new manual ticket, it is not possible to restrict the drop-down option of ticket source using a code list restriction.
SAP Cloud for Customer.
Reproducing the Issue
Go to the Service work center.
Go to the Tickets view.
Click on the New Ticket button.
You can see it default show drop down option as Manual Data Entry, Fax , Letter and Telephone.
Go to the Administrator work center.
Go to the General Setting and click on Code List Restrictions.
Click on New button and select the Business Object as Ticket.
You can see the drop down option of field Code to Restrict don't have standard Source field available.
Currently there is no standard field source available to be restricted for ticket object.
The source can not be restricted while creating new manual ticket and this is current system behavior.
Code List Restriction , Ticket Source field , KBA , new ticket , source drop down , LOD-CRM-SRP , Service Request Processing , Problem