Error sending json in POST to web API service

I'm creating a web service using Web API. I implemented a simple class

public class ActivityResult
{
    public String code;
    public int indexValue;
    public int primaryCodeReference;
}

And then I have implemented inside my controller

[HttpPost]
public HttpResponseMessage Post(ActivityResult ar)
{
    return new HttpResponseMessage(HttpStatusCode.OK);
}

But when I call the API passing in POST the file json:

{"code":"XXX-542","indexValue":"3","primaryCodeReference":"7"}

I obtain the following error message:

{
    "Message": "The request entity's media type 'text/plain' is not supported for this resource.",
    "ExceptionMessage": "No MediaTypeFormatter is available to read an object of type 'ActivityResult' from content with media type 'text/plain'.",
    "ExceptionType": "System.Net.Http.UnsupportedMediaTypeException",
    "StackTrace": "   in System.Net.Http.HttpContentExtensions.ReadAsAsync[T](HttpContent content, Type type, IEnumerable`1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)\r\n   in System.Net.Http.HttpContentExtensions.ReadAsAsync(HttpContent content, Type type, IEnumerable`1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)\r\n   in System.Web.Http.ModelBinding.FormatterParameterBinding.ReadContentAsync(HttpRequestMessage request, Type type, IEnumerable`1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)"
}

What am I doing wrong?

In the HTTP request you need to set Content-Type to: Content-Type: application/json

So if you're using fiddler client add Content-Type: application/json to the request header

1. You have to must add header property Content-Type:application/json

2. When you define any POST request method input parameter that should be annotated as [FromBody], e.g.:

   [HttpPost]
   public HttpResponseMessage Post([FromBody]ActivityResult ar)
   {
     return new HttpResponseMessage(HttpStatusCode.OK);
   }
   

3. Any JSON input data must be raw data.

another tip...where to add "content-type: application/json"...to the textbox field on the Composer/Parsed tab. There are 3 lines already filled in there, so I added this Content-type as the 4th line. That made the Post work.