AddEventHandler('rest', 'OnRestServiceBuildDescription', ['\MyRestApi', 'OnRestServiceBuildDescription']);
class MyRestApi
{
    public static function OnRestServiceBuildDescription()
    {
        return [
            'task' => [
                'task.item.add' => [
                    'callback' => [__CLASS__, 'taskAdd'],
                    'options' => [],
                ],
            ],
        ];
    }
    public static function taskAdd($query, $nav, \CRestServer $server)
    {
        if ($query['error'])
        {
            throw new \Bitrix\Rest\RestException(
                'Message',
                'ERROR_CODE',
                \CRestServer::STATUS_PAYMENT_REQUIRED
            );
        }
        if (!isset($query['id']))
        {
            return [
                'error' => 'ERROR_METHOD_NOT_FOUND',
                'error_description' => 'Method not found! ' . __LINE__,
            ];
        }
        if (!isset($query['fields']))
        {
            return [
                'error' => 'ERROR_METHOD_NOT_FOUND',
                'error_description' => 'Method not found! ' . __LINE__,
            ];
        }
        $id = intval($query['id']);
        if ($id <= 0)
        {
            return [
                'error' => 'ERROR_METHOD_NOT_FOUND',
                'error_description' => 'Method not found! ' . __LINE__,
            ];
        }
        $file = $_SERVER['DOCUMENT_ROOT'] . '/logg.log';
        $data = serialize($query);
        file_put_contents($file, $data);
        return array('result' => 'ok');
    }
}$eventManager = \Bitrix\Main\EventManager::getInstance();
$eventManager->addEventHandler(
    'tasks',
    '\Bitrix\Tasks\Task::onAfterAdd',
    ['TaskEvent', 'addCheckList']
);$field['CHECKLIST'] = [
            [
                'NODE_ID' => 'test1',
                'PARENT_NODE_ID' => 0,
                'COPIED_ID' => '',
                'PARENT_ID' => null,
                'TITLE' => 'чек-лист 1',
                'SORT_INDEX' => 0,
                'IS_COMPLETE' => 0,
                'IS_IMPORTANT' => false,
                'ACTION' => [
                    'MODIFY' => true,
                    'REMOVE' => true,
                    'TOGGLE' => true,
                ]
            ],
            [
                'NODE_ID' => 'sub_test',
                'PARENT_NODE_ID' => 'test1',
                'COPIED_ID' => '',
                'PARENT_ID' => null,
                'TITLE' => 'пункт 1',
                'SORT_INDEX' => 0,
                'IS_COMPLETE' => 0,
                'IS_IMPORTANT' => false,
                'ACTION' => [
                    'MODIFY' => true,
                    'REMOVE' => true,
                    'TOGGLE' => true,
                ]
            ],
        ];