/**
 * Implements hook_entity_type_build().
 */
function {{ machine_name }}_entity_type_build(array &$entity_types) {
  /** @var $entity_types \Drupal\Core\Entity\EntityTypeInterface[] */
  // Add a form for a custom node form without overriding the default
  // node form. To override the default node form, use hook_entity_type_alter().
  $entity_types['node']->setFormClass('mymodule_foo', 'Drupal\mymodule\NodeFooForm');
}