Insert Event to Google Calendar.dart

insertEvent(event){
try {
        clientViaUserConsent(_clientID, _scopes, prompt).then((AuthClient client){
        var calendar = CalendarApi(client);
        String calendarId = "primary";
        calendar.events.insert(event,calendarId).then((value) {
          print("ADDEDDD_________________${value.status}");
          if (value.status == "confirmed") {
            log('Event added in google calendar');
          } else {
            log("Unable to add event in google calendar");
          }
        });
        });
      } catch (e) {
        log('Error creating event $e');
      }
}

  void prompt(String url) async {

    if (await canLaunch(url)) {
      await launch(url);
    } else {
      throw 'Could not launch $url';
    }
  }

when do i use function "prompt() " ?
how i get the "url" ?

I just updated the gist. Sorry I forgot to add it earlier.

its working, thanks..., but it always prompt for access calendar, how to use without open browser or hardcode credential maybe?

thank you for the answer