Firebase Cloud Messaging giving 400 response on sending POST request for device group notification id using Retrofit library in Android

Question

I'm sending a POST request through Retrofit2 in Android to FCM creating Device Group Notification.

This is my RequestModel:

import java.util.List;
import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class FirebaseMessagingDeviceGroupRequestModel {

    @SerializedName("operation")
    @Expose
    private String operation;
    @SerializedName("notification_key_name")
    @Expose
    private String notificationKeyName;
    @SerializedName("registration_ids")
    @Expose
    private List<String> registrationIds;

    public void setNotificationKeyName(String notificationKeyName) {
        this.notificationKeyName = notificationKeyName;
    }

    public void setOperation(String operation) {
        this.operation = operation;
    }

    public void setRegistrationIds(List<String> registrationIds) {
        this.registrationIds = registrationIds;
    }

    public String getOperation() {
        return operation;
    }

    public String getNotificationKeyName() {
        return notificationKeyName;
    }

    public List<String> getRegistrationIds() {
        return registrationIds;
    }
}

This is the Response Model:

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

public class FirebaseMessagingDeviceGroupResponseModel {

    @SerializedName("notification_key")
    @Expose
    private String notificationKey;

    public String getNotificationKey() {
        return notificationKey;
    }

    public void setNotificationKey(String notificationKey) {
        this.notificationKey = notificationKey;
    }
}

This is the Retrofit2 API Client Interface:

@Headers({
        "Content-Type: application/json",
        "Authorization: key=AAAA2kYlOBY:APA91bGPnbTqQIYw21Mtyg36.......",
        "project_id: 937479...."
})
@POST("fcm/notification")
    Call<FirebaseMessagingDeviceGroupResponseModel> postNotificationGroupRequest(@Body FirebaseMessagingDeviceGroupRequestModel firebaseMessagingDeviceGroupRequestModel);

This is the MainActivity:

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("https://fcm.googleapis.com/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

RetrofitCallbacks callbacks = retrofit.create(RetrofitCallbacks.class);


FirebaseMessagingDeviceGroupRequestModel firebaseMessagingDeviceGroupRequestModel = new FirebaseMessagingDeviceGroupRequestModel();
firebaseMessagingDeviceGroupRequestModel.setNotificationKeyName("sample_group");
firebaseMessagingDeviceGroupRequestModel.setOperation("create");
firebaseMessagingDeviceGroupRequestModel.setRegistrationIds(driverKeys);

Call<FirebaseMessagingDeviceGroupResponseModel> firebaseNotificationGroupRequestCallback = callbacks.postNotificationGroupRequest(firebaseMessagingDeviceGroupRequestModel);

firebaseNotificationGroupRequestCallback.enqueue(new Callback<FirebaseMessagingDeviceGroupResponseModel>() {
    @Override
        public void onResponse(@NotNull Call<FirebaseMessagingDeviceGroupResponseModel> call, @NotNull Response<FirebaseMessagingDeviceGroupResponseModel> response) {
        if (response.body() != null) {

            driversNotificationGroupId = response.body().getNotificationKey();

            //Processing Notification Key

        } else {

            Log.e(TAG,"Response Is Empty :: " + response);
        }
    }

    @Override
    public void onFailure(@NotNull Call<FirebaseMessagingDeviceGroupResponseModel> call, @NotNull Throwable t) {
        t.printStackTrace();
     }
  });

In Log, I'm getting:

Response Is Empty :: Response{protocol=h2, code=400, message=, url=https://fcm.googleapis.com/fcm/notification

400 error code means my JSON body is not correctly formatted. How can I solve this problem? What is the correct way of sending this JSON body

JSON Body I wanna send (this is the correct one. I've tested it in Postman):

{
    "operation":"create",
    "notification_key_name":"sample_group",
    "registration_ids":["dIL7U3prTWeUHSAvpqmdVs......."]
}

I wanna send the above JSON object using retrofit2 in android app.