import retrofit2.Call;
import retrofit2.http.Field;
import retrofit2.http.FormUrlEncoded;
import retrofit2.http.POST;
public interface ServerApi {


@FormUrlEncoded
@POST("api/auth/login")
Call<Client> autoriz(@Field("mail") String mail,
                     @Field("password") String password);
}

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class LoginActivity extends AppCompatActivity {


Button btLogIn;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    btLogIn = findViewById(R.id.logIn);

    btLogIn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
              auto();
        }
    });
}

private void auto(){
    final Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("http://54.93.100.144:1337/")
            .addConverterFactory(GsonConverterFactory.create())
            .build();
    ServerApi serverApi = retrofit.create(ServerApi.class);

    Client client = new Client();
    client.mail = "ddima.ml@yandex.ru";
    client.password="qwerty";

    Call<Client> autorization = serverApi.autoriz(client.getMail(), client.getPassword());

    autorization.enqueue(new Callback<Client>() {
        @Override
        public void onResponse(Call<Client> call, Response<Client> response) {
            if(response.isSuccessful()){
            Intent intent = new Intent(LoginActivity.this, MainActivity.class);
            startActivity(intent);
            }
            else{ Log.i("Loog", "Response " +response.code());
             Log.i("Loog", "Response " +response.errorBody());}
        }

        @Override
        public void onFailure(Call<Client> call, Throwable t) {
            Toast.makeText(LoginActivity.this,"Ошибка " + t.getMessage(), Toast.LENGTH_LONG).show();
            return;
        }
    });
}}