This post assumes a reasonable knowledge of creating events in Squarespace, and creating campaigns in Mailchimp.
Create event via Squarespace - see How to create an event
Once the event has been created in Squarespace
Go to the event on the live RepowerNoosa.com site
Copy the URL for each of the Calendar links - Google & ICS, and save them in a TEXT file
In Mailchimp, make a copy of a campaign that has the RSVP and Save to Calendar buttons, as per the example image.
In your copied Mailchimp campaign
Button - add to Google Calendar - associate Google link above
Button - add to Calendar generic - associate ICS link above
Button with ICS link seems to work fine on iOS
Button with Google Cal seems to work fine on desktop if you use Google Calendar, otherwise it downloads ICS file - and when you open file it opens your default Calendar & it should then save fine