String data = "number=123&number2=321"; //данные для отправки
URL myUrl = new URL("https://url");
connection = (HttpURLConnection) myUrl.openConnection();
connection.setDoOutput(true);
connection.setDoInput(true);
connection.setRequestMethod("POST");
//указываем заголовки (как заполнить Content-Length?)
connection.setRequestProperty("Referer", "refererurl");
connection.setRequestProperty("Accept-Encoding", "gzip, deflate, br");
connection.setRequestProperty("Accept-Language", "ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4");
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
connection.setRequestProperty("Host", "host");
connection.setRequestProperty("Connection", "keep-alive");
connection.connect();
//отправляем данные
DataOutputStream wr = new DataOutputStream( connection.getOutputStream());
wr.writeBytes(data);
wr.flush();
wr.close();

InputStream in = connection.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(in, "UTF-8"));
String line = "";
String res = "";

while ((line = reader.readLine()) != null) {
                res += line;
            }
            reader.close();
            in.close();
            textView.setText(res); //выводим в textview
            connection.disconnect();