HttpClient httpclient = HttpClients.createDefault();
HttpPost httppost = new HttpPost("https://rest.sendmode.com/v2/import");
httppost.setHeader(HttpHeaders.AUTHORIZATION, YOUR_ACCESS_KEY);

String json = "{\"firstname\" : \"Hello\", \"mobilenumber\" : \"0870000000\"}";
List<NameValuePair> params = new ArrayList<NameValuePair>(1);
params.add(new BasicNameValuePair("importdata", json));
httppost.setEntity(new UrlEncodedFormEntity(params, "UTF-8"));

HttpResponse response = httpclient.execute(httppost);
HttpEntity entity = response.getEntity();