How to build request with custom body

I’m developing java library which will be interfacing one of our legacy system. All calls are via REST request. Body of each request is url encoded form.

But here is a problem, each form has single field which contains stringified JSON object with all arguments. I was wonder what will be best practice to replicate this using Retrofit library?

Currently I’m thinking about defining my request as,

Call<Response> request(@Field("payload") String payload);  

and create layer between Retrofit and request.

Is it better solution which allows me to define request which fields which are relevant for given request ie

Call<Response> request(@Field("field1") String field1,
                       @Field("field2") String field2);

and pre process them to create body required for given request