/**
 * Implements hook_entity_base_field_info().
 */
function {{ machine_name }}_entity_base_field_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'node') {
    $fields = [];
    $fields['mymodule_text'] = BaseFieldDefinition::create('string')
      ->setLabel(t('The text'))
      ->setDescription(t('A text property added by mymodule.'))
      ->setComputed(TRUE)
      ->setClass('\Drupal\mymodule\EntityComputedText');

    return $fields;
  }
}