code: def create(self, callback_url):
          
          resource = self.resource.create({'subscribed_to': 'address',
                                           'callback_url': callback_url})
          subscription = self.wrap(resource)
          self.add(subscription)
          return subscription
docs: Register a new Subscription on this collection's parent object.
  
          Args:
            callback_url (str): URI of an active endpoint which can receive
            notifications.
  
          Returns:
             A round.Subscription object if successful.