Search API: getAutocompleteSuggestions in D8?
I've solved it with Solarium

//...
use Solarium\Client;
//...

// $value is the search value from the textfield. 
if (!empty($value)) {
    $searchAPIconfig = \Drupal::config('search_api.server.server');
    $searchAPIconfigData = $searchAPIconfig-&gt;getRawData();

    $config = array(
    'endpoint' =&gt; array(
      'localhost' =&gt; array(
        'host' =&gt; $searchAPIconfigData['backend_config']['host'],
        'port' =&gt; $searchAPIconfigData['backend_config']['port'],
        'path' =&gt; $searchAPIconfigData['backend_config']['path'],
      ),
    ),
    );

    $client = new Client($config);

    if ($client !== NULL) {
        $query = $client-&gt;createSuggester();
        $query-&gt;setQuery($value);
        $query-&gt;setDictionary('mySuggester');
        $query-&gt;setOnlyMorePopular(TRUE);
        $query-&gt;setCount(10);
        $query-&gt;setCollate(TRUE);

        $resultset = $client-&gt;suggester($query)-&gt;getData();
        $results = $resultset['suggest']['mySuggester'][$value]['suggestions'];
        $suggestions = array();
        foreach ($results as $key =&gt; $termResult) {
          $suggestions[] = $termResult['term'];
        }

        //...
    }
}