code: func loginButton(r *messenger.Response) error {
  	buttons := &[]messenger.StructuredMessageButton{
  		{
  			Type: "account_link",
  			URL:  "https://" + path.Join(*publicHost, loginPath),
  		},
  	}
  	return r.ButtonTemplate("Link your account.", buttons, messenger.ResponseType)
  }
docs: // loginButton will present to the user a button that can be used to
  // start the account linking process.