Adaptive Forms Dermis Api - not able to fetch operationArguments in proxy servlet when request passed from FDM service

[pgundrat]

I have configured a FDM datasource Rets Service Type  with aem proxy servlet and created a form data model. I am trying to test my GET service at form data model. when click on test with some input data {"test1": "sample data"}. The request is going to http://localhost:4502/content/dam/formsanddocuments-fdm/my-model.executeDermisQuery.json? with the payload formData operationName=GET+%2Fservices%2Ffinance%2FcpaApproval.json&operationArguments=%7B%22test1%22%3A%22hgsdjhcfg%22%7D

1. operationName
   GET /services/finance/cpaApproval.json
2. operationArguments
   {"test1":"hgsdjhcfg"}

but when request come to my proxy servlet, I am not able to fetch operationarguement from slingRequest
I try to fetch it using req.getParameter(), req.getRequestParams() getting null all the time .

I have implemented a import com.adobe.aem.dermis.api.processor.IPreProcessor; and is invoked before my proxy servlet. In the preProcessor I am able to fetch the operationArguments from ExecutionContext as executionContext.getArguements() -> this is giving whole json. 
Can anyone please help me how to read operationArguments in proxyServlet?

is there any way to get actual request object of browser comes to executeDermiseRequest.json in the preProcessor or in proxy servlet ?