code: def AttachUserList(client, ad_group_id, user_list_id):
    
    ad_group_criterion_service = client.GetService(
        'AdGroupCriterionService', 'v201809')
  
    user_list = {
        'xsi_type': 'CriterionUserList',
        'userListId': user_list_id
    }
  
    ad_group_criterion = {
        'xsi_type': 'BiddableAdGroupCriterion',
        'criterion': user_list,
        'adGroupId': ad_group_id
    }
  
    operations = [{
        'operator': 'ADD',
        'operand': ad_group_criterion
    }]
  
    return ad_group_criterion_service.mutate(operations)['value'][0]
docs: Links the provided ad group and user list.
  
    Args:
      client: an AdWordsClient instance.
      ad_group_id: an int ad group ID.
      user_list_id: an int user list ID.
  
    Returns:
      The ad group criterion that was successfully created.