The Add to cart event is available for both CS Merchandising and CS Digital customers.
The event is not available in CS Apps.
If you use CS Merchandising, specify an SKU value identical to the value in the Product Catalogue:
If you use CS Digital only, you can omit the SKU:
To implement an Add to cart with GTM, use a custom HTML tag with this script and pass the SKU using the corresponding variable and trigger whenever there is an add to cart (usually after a select a button).