Send HTTP POST REQUEST in JSON format to the firebase API

I am developing a APP of sending PUSH , from a APP I could send a HTTP POST to API of firebase , this means that when this message arrives at the API it is entrusted to send a PUSH to all the nodes subscribed to a certain category.

The app that I used is ARC that is Chrome . I would like to attach this to code Java of android , since I would like to send notifications from an activity of my App and for that I have to send a HTTP POST REQUEST with the data in JSON . I leave you captures of how I do it with the APP .

Yesterday I was a few hours and I could not manage to send a request from android , today I have all morning and there is no way to work from android .

FROM THE APP OF THE PHOTOS I HAVE BEEN ABLE TO SEND THE PETITION AND EVERYTHING IS PERFECT:

I am using this code:

public void send_message() throws JSONException, IOException {


    String authKey = "MI_API_KEY";   // You FCM AUTH key
    String FMCurl = "https://fcm.googleapis.com/fcm/send";

    URL url = new URL(FMCurl);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setUseCaches(false);
    conn.setDoInput(true);
    conn.setDoOutput(true);
    conn.setRequestMethod("POST");
    conn.setRequestProperty("Authorization","key="+authKey);
    conn.setRequestProperty("Content-Type","application/json");

    JSONObject json = new JSONObject();
    JSONObject item = new JSONObject();
    String topic="/topics/POST";
    item.put("Titulo_", "1");
    item.put("Creador_", "2");
    item.put("Cuerpo_", "3");
    item.put("Fecha_", "4");
    item.put("Tipo_", "5");
    json.put("to", topic);
    json.put("data", item);

    OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
    wr.write(json.toString());
    wr.flush();
    conn.getInputStream();

    }