How do I send a POST request via Retrofit 2.0?

Copy and paste the results of a successful request from Postman to http://www.jsonschema2pojo.org/, if necessary, correct the received files and insert them into the Android Studio project in the (package) directory named model - the name is optional, it's just traditionally established. After that, you write two modules: the factory and the service, and put them in the project in the network directory.

Factory code:

//Импортируем необходимые классы
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

//Объявляем фабрику - только статичные поля и методы
public class ApiFactory {

    private static final String ROOT_URL = "https://your_api_address";

    static Retrofit buildRetrofit() {
        return new Retrofit.Builder()
                .baseUrl(ROOT_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }

    public static ApiService getService() {
        return buildRetrofit().create(ApiService.class);
    }
}

Service code (taking into account the POST request and the login and input parameters password):

//Импортируйте получившиеся у Вас в JSON-POJO модели
//В данном случае Вы должны назвать свою модель AccessToken
import com.YOUR_BRAND.YOUR_APP_NAME.model.AccessToken;

import retrofit2.Call;
import retrofit2.http.POST;
import retrofit2.http.Query;
import retrofit2.http.Headers;

public interface ApiService {
    @Headers("Content-Type: application/json")
    @POST("auth/login") //путь вашего сервиса к авторизации
        Call<AccessToken> getToken(
        @Query("login") String login,
        @Query("password") String password);
}

In the main part of your application, to get a token in response to a username and password, you write something like this (taking into account how you have named variables, classes, etc.):

private void getToken() {
    Call<AccessToken> call = ApiFactory.getService().getToken(EMAIL, PASSWORD);
    call.enqueue(new Callback<AccessToken>() {
        @Override
        public void onResponse(Call<AccessToken> call, Response<AccessToken> response) {
            if (response.isSuccessful()) {
                //Тут Вы просто сохраняете токен для дальнейшего использования
                MainActivity.token = response.body().getData().getToken();
                MainActivity.tokenAcquired = true;
                Log.d("myLogs", token);
                getTasks();
            } else {
                MainActivity.token = "";
                MainActivity.tokenAcquired = false;
                Log.d("myLogs", ErrorUtils.errorMessage(response));
            }
        }

        @Override
        public void onFailure(Call<AccessToken> call, Throwable t) {}
    });
}

ErrorUtils class (optional, only for parsing the error):

package com.YOUR_BRAND.YOUR_APP_NAME.network;

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.util.Locale;

import okhttp3.ResponseBody;
import retrofit2.Converter;
import retrofit2.Response;

public class ErrorUtils {

    private static final String ERROR_MESSAGE = "Код HTTP: %d\nОшибка: %s\nКод ошибки: %d";

    private static APIError parseError(Response<?> response) {
        Converter<ResponseBody, APIError> converter =
                ApiFactory.buildRetrofit()
                        .responseBodyConverter(APIError.class, new Annotation[0]);
        APIError error;
        try {
            error = converter.convert(response.errorBody());
        } catch (IOException e) {
            return new APIError();
        }

        return error;
    }

    public static String errorMessage(Response<?> response) {
        try {
            APIError error = parseError(response);
            return String.format(Locale.getDefault(), ERROR_MESSAGE, response.code(), error.message(), error.status());
        } catch (com.google.gson.JsonSyntaxException e) {
            return String.format(Locale.getDefault(), ERROR_MESSAGE, response.code(), "NULL", 0);
        }
    }
}

ApiError class

package com.YOUR_BRAND.YOUR_APP_NAME.network;

import com.google.gson.annotations.SerializedName;

public class APIError {

    @SerializedName("httpStatus")
    private int statusCode;
    @SerializedName("errorMessage")
    private String message;

    public APIError() {
    }

    public int status() {
        return statusCode;
    }

    public String message() {
        return message;
    }
}

Well, here's some information to start thinking about. com. YOUR_BRAND.YOUR_APP_NAME is the name of your project, specified in the headers of java modules. Bye sort it out and have a good night. If you have any questions, ask them, but I'll answer them tomorrow. I'm sorry in advance if there are any typos in the code. Good work!