Retrofit2: Modifying request body in OkHttp Interceptor

I using this to add post parameter to the existing ones.

 OkHttpClient client = new OkHttpClient.Builder()
                    .protocols(protocols)
                    .addInterceptor(new Interceptor() {
                        @Override
                        public Response intercept(Chain chain) throws IOException {
                            Request request = chain.request();
                            Request.Builder requestBuilder = request.newBuilder();
RequestBody formBody = new FormEncodingBuilder()
            .add("email", "[email protected]")
            .add("tel", "90301171XX")
            .build();
                            String postBodyString = Utils.bodyToString(request.body());
                            postBodyString += ((postBodyString.length() > 0) ? "&" : "") +  Utils.bodyToString(formBody);
                            request = requestBuilder
                                    .post(RequestBody.create(MediaType.parse("application/x-www-form-urlencoded;charset=UTF-8"), postBodyString))
                                    .build();
                            return chain.proceed(request);
                        }
                    })
                    .build();

public static String bodyToString(final RequestBody request){
        try {
            final RequestBody copy = request;
            final Buffer buffer = new Buffer();
            if(copy != null)
                copy.writeTo(buffer);
            else
                return "";
            return buffer.readUtf8();
        }
        catch (final IOException e) {
            return "did not work";
        }
    }

OkHttp3:

RequestBody formBody = new FormBody.Builder()
                .add("email", "[email protected]")
                .add("tel", "90301171XX")
                .build();