I got it. I have to check, if in array is "['meta_query'][0]['value']" set or not :)

add_filter('rest_test_query', function($args) {
    
    $args['meta_query'] = array(
    array(
        'key'   => 'test_1',
        'value' => esc_sql( $_GET['test_1'] ),
        )
    );
    
     //echo $args['meta_query'][0]['value'];
    //print_r($args);
    
    if (!empty($args['meta_query'][0]['value'])) {
        return $args;       
    }
    
} );

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);