Note: I always forget this stuff, so I'm putting it my blog to help me remember it.

Calling a JSON REST based service with some params isn't that hard.  I have an endpoint that has this interface:

        [WebInvoke(UriTemplate = "/Login",
            Method="POST",
            BodyStyle = WebMessageBodyStyle.Wrapped,
            RequestFormat = WebMessageFormat.Json,
            ResponseFormat = WebMessageFormat.Json )]
        [OperationContract]
        bool Login(LoginData ld);

The LoginData class is defined like this:

    [DataContract]
    public class LoginData
    {
        [DataMember]
        public string UserName { get; set; }
        [DataMember]
        public string PassWord { get; set; }
        [DataMember]
        public string AppKey { get; set; }
    }
 

Now that you see my method to call to login as well as the class that is passed for the login, the body of the login request looks like this:

{ "ld" : {  "UserName":"testuser", "PassWord":"ackkkk", "AppKey":"blah" } }

The header (in Fiddler), looks like this:

User-Agent: Fiddler
Host: hostname
Content-Length: 76
Content-Type: application/json

And finally, my url to POST against is:

http://www.something.com/...../someservice.svc/Login