Use following NPM package to add a calendar thing: https://www.npmjs.com/package/add-to-calendar-button.
Make sure to have pnpm installed, then:
# pnpm install
# pnpm start
Open the page on http://localhost:1234.
For the lazy fucks that doesn't want to pull it down, check it out here instead: https://me.wiberg.tech/add-to-calendar/.