/**
 * Implements hook_entity_bundle_field_info().
 */
function {{ machine_name }}_entity_bundle_field_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
  // Add a property only to nodes of the 'article' bundle.
  if ($entity_type->id() == 'node' && $bundle == 'article') {
    $fields = [];
    $fields['mymodule_text_more'] = BaseFieldDefinition::create('string')
      ->setLabel(t('More text'))
      ->setComputed(TRUE)
      ->setClass('\Drupal\mymodule\EntityComputedMoreText');
    return $fields;
  }
}