@adam.dickson, you actually don't obtain client session ID, you define your own, by adding an item to the request's HTTP header.This header is optional, and only useful for debugging, it has no other purpose. It allows the DSS team to investigate specific queries, identified by your specific session IDs. In production one usually does not add this header.
For the value, you can set whatever you like, as long as you make it unique for every single query we must debug.
In Postman it would look like this:
This is explained in more detail in the Diagnostics help page.