How to send a custom Solr Distance Query with SearchApi
Debugging around I found the solution:

$index = Index::load('default_solr_index');
/* @var \Drupal\search_api\Query\Query $query */
$query = $index-&gt;query();
$query-&gt;keys($form_state-&gt;getValue("keys"));
$location_options = (array) $query-&gt;getOption('search_api_location', []);

$location_options[] = [
      'field' =&gt; "field_geo_koordinaten",
      'lat' =&gt; 48.1371079,
      'lon' =&gt; 11.5753822,
      'radius' =&gt; 50,
    ];

$query-&gt;setOption('search_api_location', $location_options);
$results = $query-&gt;execute();


Thanks for pointing me in the right direction.