OpenApi : How to add variable for some attribut

Hello everyone,

In the life cycle of my API, I have to contact a partner to whom I send a token and a callback url. After having performed his operations, he must then contact me on this callback url by transmitting the given token.
In order to test my API, I try to mock my partner using Prism. I describe my partner using openapi: 3.1.0.

So I describe the requestBody of my partner by quoting the different parameters. "callbackUrl" represents the callback url on which my partner must call me back and "requestId" represents the return token.

I also describe the callback using the variable in the requestBody to describe the url

Everything is fine so far but I would like the "requestId" generated by the callback body to contain the value of the "requestId" of my initial request.
Basically I would like to do something like this:

Unfortunately it does not work as I would like...
Does anyone know how to do this please?