/**
 * Implements hook_entity_field_values_init().
 */
function {{ machine_name }}_entity_field_values_init(\Drupal\Core\Entity\FieldableEntityInterface $entity) {
  if ($entity instanceof \Drupal\Core\Entity\ContentEntityInterface && !$entity->foo->value) {
    $entity->foo->value = 'some_initial_value';
  }
}