REST Service Error 400 when passing entity record type parameter

Good day,
I created a REST Service (POST) with string parameter (Body) - working fine. 
When I change the parameter to an Entity Record, it fails with error: 400 Bad Request - Failed to parse JSON request content.
The service is called from a .NET window service using JSON .Net and RESTSharp
var restClient = new RestClient(Properties.Settings.Default.WebHookEndPoint);
restClient.Authenticator = new HttpBasicAuthenticator("Username", "pwd");
var request = new RestRequest(Method.POST);
request.Resource = "/Create_My_Transaction";
request.RequestFormat = DataFormat.Json;
var response = restClient.Execute(request);
The content of the message parameter is as follows (same name, order and data types as the attributes):
{"Param1":800,"Param2":800," Param3":"ZAR"," Param4":"MQ Test Transaction"," Param5":"555**555","Param6":"2016-03-13T10:35:47.3865167+02:00","Param7":"2016-03-13T10:35:47.3865167+02:00","Param8":123.7}
The class:
    public class DummyData
        public int Param1 { get; set; }
        public int Param2 { get; set; }
        public string Param3 { get; set; }
        public string Param4 { get; set; }
        public string Param5 { get; set; }
        public DateTime Param6 { get; set; }
        public DateTime Param7 { get; set; }
        public Double Param8 { get; set; }
Any assistance will be appreciated!
Thank you!
Hi Mariaan,

The problem seems to be on the .NET side, not the OutSystems side? In that case, this is not the right forum to ask questions about it.