I got that working :)

    add_filter('rest_myRoute_query', function($args, $request) {
      $fields = array('myCustomField');
    
      foreach ($fields as $field) {
        if (! empty($request[$field])) {
          $args['meta_query'][] = array(
            'key'   => $field,
            'value' => esc_sql($request[$field]),
          );
        }
      }
    
      return $args;
    }, 10, 2);