How to store a request JSON in a variable and use it in the mocked response body

I want to mock exact request values in response.

ex :

request body

{“username”:“google”,
“address” : {“city”:“CA”},
“contactNo”:“8274287844”
}

response should be like this : based on request i need to map exact values to same key(from request to response - mocking with example)

{“username”:“google”,
“address”:{“city”:“CA”}
}

I have tried adding changes in prescript

var reqBody= pm.request.body.raw
console.log(reqBody)
//console.log(req)
var json_verify = function(reqBody){ try { JSON.parse(reqBody); return true; } catch (e) { return false; }}
if(json_verify){
    var val = JSON.parse(pm.request.body.raw)
    var bol = true
    for(var key in val)
    {
     if(key === "username"){
       pm.globals.set("userName", key);
     }
   }
}

my example mocked response body like this :

{"username":{{userName}}} or tried {"username":{{$userName}}} or tried

{"username":pm.globals.usrName}

but when i ran response body is coming as :

{"username":{{userName}}} or {"username":{{$userName}}} or {"username":pm.globals.userName}

expected output:

{“username”:“google”}

please suggest me the possible ways. any help much appreciated