code: public function addMediaToFolder($folderId, array $mediaIds)
      {
          $parameters = [
              'query' => [],
              'body' => json_encode(['mediaIds' => $mediaIds], JSON_UNESCAPED_UNICODE),
              'headers' => [],
          ];
  
          $result = $this->post('v1/folders/' . $folderId . '/media', $parameters);
  
          return $result;
      }
docs: Add Media to Folder.
  
  @param  int   $folderId folder to add media to
  @param  int[] $mediaIds an array of int values
  @return array