code: public ItemRequest<Task> addSubtask(String task) {
      
          String path = String.format("/tasks/%s/subtasks", task);
          return new ItemRequest<Task>(this, Task.class, path, "POST");
      }
docs: Creates a new subtask and adds it to the parent task. Returns the full record
  for the newly created subtask.
  
  @param  task The task to add a subtask to.
  @return Request object