code: func (m *Messenger) GetPSID(token string) (*string, error) {
  	resp, err := m.doRequest("GET", fmt.Sprintf(GraphAPI+"/v2.6/me?fields=recipient&account_linking_token=%s", token), nil)
  	if err != nil {
  		return nil, err
  	}
  	defer resp.Body.Close()
  	read, err := ioutil.ReadAll(resp.Body)
  	if resp.StatusCode != http.StatusOK {
  		er := new(rawError)
  		json.Unmarshal(read, er)
  		return nil, errors.New("Error occured: " + er.Error.Message)
  	}
  	acc := new(accountLinking)
  	return &acc.Recipient, json.Unmarshal(read, acc)
  }
docs: // GetPSID fetches user's page scoped id during authentication flow
  // one must supply a valid and not expired authentication token provided by facebook
  // https://developers.facebook.com/docs/messenger-platform/account-linking/authentication