public class UserRequest<T> where T : RealmObject
    {
        public String userName { get; set; }
        public String password { get; set; }
        public String userId { get; set; }
        public int page { get; set; }
        public IList<T> requestList { get; set; }

    }

UserRequest<Contact> userRequest = new UserRequest<Contact>();
userRequest.userId = "112";
userRequest.requestList = contactList;

{

  "userId": "112",
  "requsetList": [
    {

      "firstName": "john",
      "lastName": "j",
      "contactNumber": "(992) 414-9999",

    }
  ]
}

UserRequest userRequet = new UserRequest();
userRequest.userName ="abc";
userRequest.password="1234";