gistfile1.txt

public interface ApiClientInterface {
    @Multipart
    @POST("xxxx")
    Call<ResponseBody> serviceLogin(@PartMap Map<String, RequestBody> params);
}

..........

        ApiClientInterface mApiClientInterface= ApiClient.getInstance().create(ApiClientInterface.class);
        RequestBody body = RequestBody.create(MediaType.parse("text/plain"), "value");
        Map<String, RequestBody> requestBodyMap = new HashMap<>();
        requestBodyMap.put("key", body);
        Call<ResponseBody> response=mApiClientInterface.serviceLogin(requestBodyMap);
        userRCall.enqueue(................);
        

Send form-data in retrofit

        
        
public class ApiClient {
    private static final String URL = "";
    private static Retrofit mRetrofit = null;

    public static Retrofit getInstance() {
        if (mRetrofit == null) {
            mRetrofit = new Retrofit.Builder()
                    .baseUrl(URL)
                    .build();
        }
        return mRetrofit;
    }
}

string that you need to server...
don't forget to use scalar converter in your retrofit