//Method: POST
//Parametes: box.content, box.labels, box.ext, box.xid
//box.tags: length<=220; in-line;
//box.ext: in-line;
//box.xid: length<=24; in-line;
//Return: {"ret": true, "sid": "%sid%", "echo": ""} or {"ret": false, "sid":"", "echo": "error"}