You can view an example of this working of this functionality at the Zesty Burger Demo.

Create the endpoint

Create an endpoint with any relevant name and a .ics extension. In this example we used event.ics.

Write the ICS file

Below is an example ICS syntax wrapped with a parsley each loop, and replacing dynamic pieces of content with parsley calls to relevant fields:

Note that the query in the each loop is looking for a query parameter names id to get the correct event.

Add the Markup

Here is the HTML and parsley we are using on the event page to generate the button where we are referencing the end point we created and adding in the event id as a query parameter.