threadsettings.go
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
}
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
}
// 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