Check for requested domain in autoload.php in multisite installation

Hello,

I have a multisite installation with Contao 3.5.*
I wanted to load different classes in my module in config/autoload.php based on the requested domain.

PHP Code:

---

if ('http://subdomain.domain.tld' == \Environment::get('url')) {
    ClassLoader::addClasses(array
    (
        'Contao\FormRadioExplain' => 'system/modules/MODULE/FormRadioExplain.php',
    ));
} 


---

but the class \Environment is not yet available at this point. ("Class not found")

QUESTION: How do I use the Environment class in autoload.php correctly OR how do I check reliably which site is being requested?