Media library extra form widget is not working with gallery media

I already found a hint to my issue, see hook_media_source_info_alter() in media_library.module (media_library_media_source_info_alter()): the form to create media entities got added there for some types. For the slideshow plugin (docroot/modules/contrib/media_entity_slideshow/src/Plugin/media/Source/Slideshow.php) the media_library_add is not defined. Therefore \Drupal\media_library\MediaLibraryUiBuilder::buildMediaTypeAddForm() returns an empty array:

if (empty($plugin_definition['forms']['media_library_add'])) {
  return [];
}

Therefore the next step would be to add "add forms" for the other media plugins.

I have found how it might work: Add the media_library_add form to source info using hook_media_source_info_alter():

/**
 * Implements hook_media_source_info_alter().
 */
function custom_module_media_source_info_alter(array &$sources) {
  if (empty($sources['podcast']['forms']['media_library_add'])) {
    $sources['podcast']['forms']['media_library_add'] = MediaNameForm::class;
  }
}

And then implement the form, see for example docroot/core/modules/media_library/src/Form/OEmbedForm.php