Let's try to call a post REST WCF service from Fiddler. Starting from the WCF Service definition
[WebInvoke(UriTemplate = "/clients",
    BodyStyle = WebMessageBodyStyle.Wrapped,
    Method = "POST")]
[OperationContract]
ClientDto Save(ClientData client);
Where ClientData is as follows:
[DataContract]
public class ClientData
{
    public int Id {get; set;}
    public string Name {get; set;}
}
In fiddler, chose "Composer" tab. and then add the service url, and in the request add : "Content-Type: application/json"

and in the request body, add the following:

And that is it