JAVA: Design a service which let's clients override unique key generation logic

I am working on developing a service as a platform. This service stores record for different clients.
A client can have different fields which defines it's uniqueness.
Eg.
employeeId or
employeeId + department or
employeeId + department + location.

How can I design such usecase, which let's each new on-boarded client define uniqueness of their data and my service performs CRUD operation using those unique keys at runtime.

Thanks.