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');
}


/**
  * Implements hook_form_alter().
  */
// node_simplenews_issue_edit_form
// function materio_simplenews_form_alter(&$form, &$form_state, $form_id) {
function materio_simplenews_form_node_simplenews_issue_form_alter(&$form, &$form_state, $form_id) {
  if(isset($_REQUEST['newsletter'])){
    $news_id = $_REQUEST['newsletter'];
    $newsletters = array_keys(simplenews_newsletter_get_all());
    if(in_array($news_id, $newsletters)){
    
      // prepopulate type of news
      $form['simplenews_issue']['widget']['target_id']['#default_value'] = [$news_id];
      $form['simplenews_issue']['widget']['target_id']['#disabled'] = true;
      
      // change default template regarding type of news
      $form['body']['widget'][0]['#default_value'] = materio_simplenews_getSimplenewsNodeBodyTemplate($news_id);
      $form['body']['widget'][0]['#rows'] = 50;
    }
  }

}

function materio_simplenews_getSimplenewsNodeBodyTemplate($news_id){
  return file_get_contents(drupal_get_path('module', 'materio_simplenews').'/templates/simplenews_'.$news_id.'_node.html');
}