code: public void setElementDefaultAction(Integer index, String type, String url, Boolean messenger_extensions, String webview_height_ratio, String fallback_url)
      {
          this.elements.get(index).put("default_action_type", type);
          this.elements.get(index).put("default_action_url", url);
          this.elements.get(index).put("default_action_messenger_extensions", String.valueOf(messenger_extensions));
          this.elements.get(index).put("default_action_webview_height_ratio", webview_height_ratio);
          this.elements.get(index).put("default_action_fallback_url", fallback_url);
      }
docs: Set Element Default Action
  
  @param index the element index
  @param type the element type
  @param url the element url
  @param messenger_extensions the messenger extensions
  @param webview_height_ratio the webview height ratio
  @param fallback_url the fallback url