how to parameterize the value in json file - loadrunner

Need some help in loadrunner scripting with REST api. I have a requirement that LR script should always replace the unique parameter and this parameter should be part of Json body. File whatever i am using is huge. Hence, i created payload.json in extra files of LR. In Bodyfilepath i give this name of json file. In the payload.json i have parameterised a value (which needs to be unique every iteration) for request to be succesful. However, this paramter value is not getting replaced. Can anyone help me or share the code that helps to replace the value in the json file with the parameter value Thank you