Adding middleware to spark route

How can I add middleware to spark routes such as register?

Level 1

$router->group(['middleware' => ['web', 'system']], function ($router) {

	$router->get('/register', '\Laravel\Spark\Http\Controllers\Auth\RegisterController@showRegistrationForm')->name('register');
    $router->post('/register', '\Laravel\Spark\Http\Controllers\Auth\RegisterController@register');

});

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