http://localhost:8838/validatelogin?username={username}&Password={password}&userid={userid}

var client = new RestClient("http://localhost:8838/validatelogin");
var request = new RestRequest("username={username}&Password={password}&userid={userid}", Method.POST);
request.AddParameter("userName", "username", ParameterType.UrlSegment);
request.AddParameter("password", "password", ParameterType.UrlSegment);
request.AddParameter("userid", "1234", ParameterType.UrlSegment);

var response = client.Execute(request);