code: 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
  }
docs: // 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