How Does the Agent Know Which User is Using it?

Summary:

I am working on creating an agent in HCM that would help to update a user's personal information. While looking in all of the REST API docs, most of the variable require the person's person number/id. How would the system get that information, and how would I be able to utilize that information to the do all of the queries that I want to do? Or is there a best practice for this to ensure that the agent has/gets all of the information required to proceed with the rest of the calls.

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