When a Scripted REST API has Read - Only Protection Policy, the ability to customize it is limited.
The Scripted REST APIs provided on the platform are Scoped Applications and are read-only protected to prevent customization of them.
In order to customize them you need to create your own scripts with 'None' protection policy.
The endpoints link to script include API that provides logic customization points via override of script include functions e.g. ChangeProcess for Change Management is an empty script include that can have custom logic overrides and ChangeProcessSNC which is SNC shipped logic to be overriden. However, the Scripted REST Service and Scripted REST Resource are read-only to prevent customization.
The workaround is to create your own Scripted REST Service and Scripted REST Resource records in Global scope.