How to send json data from Android to Asp.net

Help me to post json data from Android to Asp.net Here is my code below. Android sending json data to webapi

public class UploadByTableActivity extends Activity {

    final static String url = "https://webapi.com/api/post/";

    HttpResponse response;
    AsyncHttpClient client;

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

        Button btnWO = (Button) findViewById(R.id.btnwo);

        btnWorkOrder.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                JSONObject jsonParams = new JSONObject();
                String username = "id";
                String password = "pw";
                client =  new AsyncHttpClient(true, 80, 443);
                client.addHeader(
                        "Authorization",
                        "Basic " + Base64.encodeToString(
                                (username + ":"+ password).getBytes(),Base64.NO_WRAP)
                );
                client.setEnableRedirects(true);
                RequestParams params = new RequestParams();
                //  params.put("file", new File(pathoffile));
                params.put("name", "name1");
                params.put("phone", "111-111-1111");

                StringEntity entity = null;
                try {
                    entity = new StringEntity(params.toString());
                } catch (UnsupportedEncodingException e) {
                    e.printStackTrace();
                }

                client.post(null,url,entity, "application/json", new AsyncHttpResponseHandler(Looper.getMainLooper()) {

                    @Override
                    public void onSuccess(int i, Header[] headers, byte[] bytes) {
                        Log.i("appTag", "OK");
                    }

                    @Override
                    public void onFailure(int i, Header[] headers, byte[] bytes, Throwable throwable) {
                        Log.i("appTag"," Error = " + throwable.getMessage());
                    }
                });
            }
        });
    }

Try to retrieve jsondata from webapi to asp.net but jsonString is null. Please help me how to get json data.

[HttpPost]
public string post([FromBody]string jsonString){
    return jsonString; 
}