Add subdomain parameter in api routes Laravel

I'm trying to add parameter subdomain in routes in Laravel 5.8

Here is my code:

Route::domain('{subdomain}.localhost:8080')->group(function () {

    Route::group(['prefix' => 'auth'], function () {
        Route::post('login', 'Api\Auth\LoginController@login');
        Route::post('register', 'Api\Auth\RegisterController@register');
    });
});

But I get this error:

Help me to understand what am I missing!

via Chebli Mohamed