code: def add_timeline_to_sketch(self, sketch_id, index_id):
      
      resource_url = '{0:s}/sketches/{1:d}/timelines/'.format(
          self.api_base_url, sketch_id)
      form_data = {'timeline': [index_id]}
      self.session.post(resource_url, json=form_data)
docs: Associate the specified timeline and sketch.
  
      Args:
        sketch_id (int): ID of sketch
        index_id (int): ID of timeline to add to sketch