code: func (c *Manager) AttachTag(ctx context.Context, tagID string, ref mo.Reference) error {
  	id, err := c.tagID(ctx, tagID)
  	if err != nil {
  		return err
  	}
  	spec := internal.NewAssociation(ref)
  	url := internal.URL(c, internal.AssociationPath).WithID(id).WithAction("attach")
  	return c.Do(ctx, url.Request(http.MethodPost, spec), nil)
  }
docs: // AttachTag attaches a tag ID to a managed object.