[Duplicate] Add to Calendar Button, using Javascript

I would like to add a button to my form that will allow the user to click and have the event information sent to their iCal - or any other calendar they may be using.

I'm not a Javascript expert. Here is some code I found - but it doesn't work.
If there is anyone out there who can help it will be greatly appreciated.

var cal = ics();

  cal.addEvent('Demo Event', 'This is an all day event', 'Port Elizabeth', '10/10/2018', '10/10/2018');

  cal.addEvent('Demo Event', 'This is thirty minute event', 'Port Elizabeth', '10/10/2018 5:30 pm', '10/10/2018 6:00 pm');

<a href="javascript:cal.download()">Test</a>