Post Json request Unity web

I want to post a request using json to online web services.

This is my code:

 JSONObject j = new JSONObject ();
 
     j.AddField ("Id", "1234567890");
     j.AddField ("MagicKey", ApplicationServices.magicKey); 
     j.AddField ("RequestedBy", "09996f84-1a06-e211-a518-001aa020d699");
     j.AddField ("Timestamp", "/Date(1547535370953)/"); 
     j.AddField ("RequestSource", "Person");
     j.AddField ("RequestedGameId", "375b43c0-91be-e011-a505-001aa020d699"); 
     j.AddField ("RequestedPersonId", "09996f84-1a06-e211-a518-001aa020d699");
     string json = j.ToString ();
 
     Dictionary<string, string> header = new Dictionary<string, string>();
     header.Add ("Accept", "application/json");
     header.Add ("Content-Type", "text/json");
 
     byte[] encode = Encoding.ASCII.GetBytes (json.ToCharArray ());
 
 
     WWW getConfig = new WWW ("http://192.168.0.140/services/scoreupload.svc/json/GetGameConfigurationLite", encode, header);

This however keeps returning "500 Internal Server Error"

Why? and how do I make it work?