Retrofit: How to make XML request and get back JSON response

found the answer myself, actually it was SUPER EASY.

APIService Class:(RequestBody is the key here)

import okhttp3.RequestBody;
import retrofit2.Call;
import retrofit2.http.Body;
import retrofit2.http.POST;

public interface RetrofitAPIService {
//    @Headers("Content-Type: application/json")
    @POST("/webservice.asmx?op=Login")
    Call<RetrofitResponseBody> login(@Body RequestBody body);
}

note: the key for the following code is this line:

RequestBody requestBody = RequestBody.create(MediaType.parse("text/xml"), requestBodyText);

MainActivity.java:

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

        EditText userName = findViewById(R.id.userName);
        EditText password = findViewById(R.id.password);

        RetrofitAPIService mAPIService = RetrofitAPIUtils.getRetrofitAPIService();

        String requestBodyText = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n" +
                "<soap12:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:soap12=\"http://www.w3.org/2003/05/soap-envelope\">\n" +
                "  <soap12:Body>\n" +
                "    <Login xmlns=\"http://tempuri.org/\">\n" +
                "      <username>" + userName.getText() + "</username>\n" +
                "      <password>" + password.getText() + "</password>\n" +
                "    </Login>\n" +
                "  </soap12:Body>\n" +
                "</soap12:Envelope>";
        RequestBody requestBody = RequestBody.create(MediaType.parse("text/xml"), requestBodyText);
        Call<RetrofitResponseBody> response = mAPIService.login(requestBody);

        response.enqueue(new Callback<RetrofitResponseBody>() {
            @Override
            public void onResponse(Call<RetrofitResponseBody> call, Response<RetrofitResponseBody> response) {
                try {
                    Log.d("JavadR:",
                            response.body().getUserID().toString() +
                                    " - " +
                                    response.body().getFailureText()
                    );
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onFailure(Call<RetrofitResponseBody> call, Throwable t) {

            }
        });

    }

the RetrofitResponseBody is not of too much importance but for consistency and convenience I'll post it here:

import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class RetrofitResponseBody {

    @SerializedName("UserID")
    @Expose
    private Integer userID;
    @SerializedName("FailureText")
    @Expose
    private Object failureText;
    @SerializedName("UserValidPasswordCode")
    @Expose
    private Integer userValidPasswordCode;
    @SerializedName("UserPatientIsActiveWithNationalIDCode")
    @Expose
    private Boolean userPatientIsActiveWithNationalIDCode;

    public Integer getUserID() {
        return userID;
    }

    public void setUserID(Integer userID) {
        this.userID = userID;
    }

    public Object getFailureText() {
        return failureText;
    }

    public void setFailureText(Object failureText) {
        this.failureText = failureText;
    }

    public Integer getUserValidPasswordCode() {
        return userValidPasswordCode;
    }

    public void setUserValidPasswordCode(Integer userValidPasswordCode) {
        this.userValidPasswordCode = userValidPasswordCode;
    }

    public Boolean getUserPatientIsActiveWithNationalIDCode() {
        return userPatientIsActiveWithNationalIDCode;
    }

    public void setUserPatientIsActiveWithNationalIDCode(Boolean userPatientIsActiveWithNationalIDCode) {
        this.userPatientIsActiveWithNationalIDCode = userPatientIsActiveWithNationalIDCode;
    }

}