If you are using Parus basic template then api is already included by default.

// If you use a module itself, just include the api somewhere on the app bootstrap. 
require(__DIR__ . '/../vendor/rokorolov/parus/src/admin/api/api.php');
require(__DIR__ . '/../vendor/rokorolov/parus/src/admin/api/api_presenter.php');
require(__DIR__ . '/../vendor/rokorolov/parus/src/admin/api/api_cache.php');