materio-d8/web/modules/custom/materio_simplenews/materio_simplenews.module

<?php

function materio_simplenews_entity_type_alter(array &$entity_types) {
  $entity_types['simplenews_subscriber']->setFormClass('materio_block', '\Drupal\materio_simplenews\Form\MaterioSubscriptionsBlockForm');
}