I've managed to create a search form against my Solr Server with views. But I want to do the request by myself (having my own form and submit method). There's a solution for D7 here: https://drupal.stackexchange.com/questions/74505/how-to-send-a-custom-solr-query-using-search-api-solr but none of that seems to be usable with the D8 modules.

I believe there must be something like 

    $result = SolrSearchService('myServer', 'myIndex')->request(['title' => 'Test']);

But I can not find anything.