code: func (m *Messenger) SetGreetingText(text string) error {
  	if len(text) > 160 {
  		return fmt.Errorf("Greeting text is too long. It has to be at most 160 characters long.")
  	}
  
  	result, err := m.changeThreadSettings(http.MethodPost, &threadSettings{
  		Type:     settingTypeGreeting,
  		Greeting: &greeting{Text: text},
  	})
  	if err != nil {
  		return err
  	}
  	if result.Result != "Successfully updated greeting" {
  		return fmt.Errorf("Error occured while setting greeting, invalid result: %s", result.Result)
  	}
  	return nil
  }
docs: // SetGreetingText sets a greeting text which is only rendered the first time user interacts with the Page on Messenger
  // https://developers.facebook.com/docs/messenger-platform/thread-settings/greeting-text
  // text must be UTF-8 and have a 160 character limit