How can I format my cart URL in the google checkout settings to allow customers to add products directly to the cart?

I have a google merchant account and see that I can provide a URL to my cart so that customers can buy items in few steps by adding them directly from google. I’ve tried a bunch of different URL variations to try to get it to work but am at a loss.

I’ve tried https://www.keystonecandle.com/checkout/cart?product_Id={id}, https://www.keystonecandle.com/checkout/cart?productId={id}, https://www.keystonecandle.com/checkout/cart?product.sku={id}

among some others. When I click on the Test URL structure, it loads up the cart page and shows the SKU in the URL like this, for example… https://www.keystonecandle.com/checkout/cart/?productId=Tin8-Pumpk however nothing is added to the shopping cart

does anyone know how to make this work? My Magento store doesn’t show anything after /cart when I add products directly on my site so I’m not sure what the correct variable should be.

Thanks.