JSONObject jsonObj = new JSONObject();
jsonObj.put("username", username);
jsonObj.put("apikey", apikey);
// Create the POST object and add the parameters
HttpPost httpPost = new HttpPost(url);
StringEntity entity = new StringEntity(jsonObj.toString(), HTTP.UTF_8);
entity.setContentType("application/json");
httpPost.setEntity(entity);
HttpClient client = new DefaultHttpClient();
HttpResponse response = client.execute(httpPost);

curl -l -H "Content-type: application/json" -X POST -d '{"phone":"13521389587","password":"test"}' http://domain/apis/users.json

$.ajax({
 url:url,
 type:"POST",
 data:data,
 contentType:"application/json; charset=utf-8",
 dataType:"json",
 success: function(){
  ...
 }
})

$data = array("name" => "Hagrid", "age" => "36");                                   
$data_string = json_encode($data);    
$ch = curl_init('http://api.local/rest/users');    
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");              
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string); 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); 
curl_setopt($ch, CURLOPT_HTTPHEADER, array(          
  'Content-Type: application/json', 
  'Content-Length: ' . strlen($data_string))      
);                                                           
$result = curl_exec($ch);