threadsettings.go
func (m *Messenger) SetGetStartedButton(payload string) error {
	result, err := m.changeThreadSettings(http.MethodPost, &threadSettings{
		Type:          settingTypeCallToActions,
		State:         threadStateNew,
		CallToActions: []ctaPayload{ctaPayload{Payload: payload}},
	})
	if err != nil {
		return err
	}
	if result.Result != "Successfully added new_thread's CTAs" {
		return fmt.Errorf("Error occured while setting get started button, invalid result: %s", result.Result)
	}
	return nil
}
func (m *Messenger) SetGetStartedButton(payload string) error {
	result, err := m.changeThreadSettings(http.MethodPost, &threadSettings{
		Type:          settingTypeCallToActions,
		State:         threadStateNew,
		CallToActions: []ctaPayload{ctaPayload{Payload: payload}},
	})
	if err != nil {
		return err
	}
	if result.Result != "Successfully added new_thread's CTAs" {
		return fmt.Errorf("Error occured while setting get started button, invalid result: %s", result.Result)
	}
	return nil
}
// SetGetStartedButton sets a button which is shown at the bottom of the window ans is only rendered the first time the user interacts with the Page on Messenger
// When this button is tapped, we will trigger the postback received callback and deliver the person's page-scoped ID (PSID).
// You can then present a personalized message to greet the user or present buttons to prompt him or her to take an action.
// https://developers.facebook.com/docs/messenger-platform/thread-settings/get-started-button