Save a value from mock server call

I have a mock server that is receiving messages I send from my system. I can see a JSON structure I send in Mock Server Call Log, inside the Request Body.
How can I save a value from this Request Body somewhere e.g. as environment or collection variable?
I need to save a specific value from the Request Body and use it to inside a Response Body.
Currently, Response body is sending hardcoded values I set up myself. I need to be able to save value from Request body and use it in the Response body.

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