How to add an entity reference field to user entity programmatically.

I need to add an entity reference filed to user entity programmatically. How this can be done? I checked the entity_reference_test.module and found following code snippet:

use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;

/**
 * Implements hook_entity_base_field_info().
 */
function entity_reference_test_entity_base_field_info(EntityTypeInterface $entity_type) {
  $fields = array();

  if ($entity_type->id() === 'entity_test') {
    $fields['user_role'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('User role'))
      ->setDescription(t('The role of the associated user.'))
      ->setSetting('target_type', 'user_role')
      ->setSetting('handler', 'default');
  }

  return $fields;
}

But seems this does not work. Please help me to figure out this.
Any help will be highly appreciated.

Thanks in Advance.