Hi Priya,
"Kindly let me know if any workaround to pass the request body in JSON format to the service. "
- There are not many ways I am aware of. You could theoretically try doing something in the OnBeforeRequest, but it would be confusing and I don't see a reason how it could make the things better.
In the end, if the Service you are consuming is written this way, there is no way around it. Just pass the parameters/structures to the wrapping Server/Service Action as input and inside populate the REST call however you should in your case.;)