Secure - injecting data to a parameter in a REST request

Hi

I have a REST service and my request is as follows

{
"serviceHeader": {
"step": "",
"token": ""
},
"params": {
"User": ""
}
}

How should i do to map the injection to the field User?

I have tried to add in the configuration parameters the xpath $.params.User, but it doesnt work, probably as it is a json

Any idea of how could i make this work?

Thanks