<uses-permission android:name="android.permission.INTERNET" />

public class PostRequest extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... strings) {

        String address = strings[0];
        String username = strings[1];
        String password = strings[2];

        try {
            URL obj = new URL(address);
            HttpURLConnection con = (HttpURLConnection) obj.openConnection();
            con.setRequestMethod("POST");
            con.setRequestProperty("Accept-Language", "en-US,en;q=0.5");
            String urlParameters = "username=" + username + "&password=" + password;
            con.setDoOutput(true);
            DataOutputStream wr = new DataOutputStream(con.getOutputStream());
            wr.writeBytes(urlParameters);
            wr.flush();
            wr.close();
            BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();
            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();
            return response.toString();
        } catch (Exception e) {
            // catch exceptions
        }
        return null;
    }
}

String responce =  new PostRequest().execute("http://www.example.com/script.php", "username", "password").get();