code: func (a *PostbackAction) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Type        ActionType `json:"type"`
  		Label       string     `json:"label,omitempty"`
  		Data        string     `json:"data"`
  		Text        string     `json:"text,omitempty"`
  		DisplayText string     `json:"displayText,omitempty"`
  	}{
  		Type:        ActionTypePostback,
  		Label:       a.Label,
  		Data:        a.Data,
  		Text:        a.Text,
  		DisplayText: a.DisplayText,
  	})
  }
docs: // MarshalJSON method of PostbackAction