Adding an Add to Calendar link to a Carrd site works on all plans - use a Button element with your AddEvent link as the URL.

Step-by-step

1. In your AddEvent dashboard, open your event and go to Sharing
2. Copy the Add to Calendar link
3. In Carrd's editor, add a Button element. Set the label to "Add to Calendar" and paste the AddEvent URL as the button link
4. Visitors click the button and choose their calendar app

Carrd tips

The Button element approach works on all Carrd plans including free. For the full interactive dropdown picker (where the calendar picker appears inline on the page), you need a Carrd Pro plan with Embed support - paste the AddEvent button embed code into an Embed element.