When I upload a string (json) to the API I allways get Null.

Client:

            WebClient Client = new WebClient();
            Client.Encoding = System.Text.Encoding.UTF8;
            Client.Headers[HttpRequestHeader.ContentType] = "application/json; charset=UTF-8";
            Client.Headers[HttpRequestHeader.Accept] = "application/json";

            var mLink = "http://localhost:60760/Umbraco/Api/WebApi/CreateUser";

            dynamic json = new JObject();
            json.FirstName = "Lars";
            json.LastName = "Rasmussen";

            string mBack = Client.UploadString(mLink, "POST", json.ToString());

API:

public class WebApiController : UmbracoApiController
{
    // ~/Umbraco/Api/WebApi/CreateUser/
    public IEnumerable<WebApiCreateUser> CreateUser([FromBody] string JsonData)
    {
       // JsonData is allways NULL why? Have do I get the json string?

        List<WebApiCreateUser> ls = new List<WebApiCreateUser>();
        ls.Add(new WebApiCreateUser() { Message = "Succes" });          
        return ls;
     }  }

Thanks for any help :-)