It's just {{your_key}}

So, in your case, it's: {{product_name}}, {{product_SKU}}, {{created_date}}

I haven't tested yet to see whether objects are passed with object notation, i.e. if we pass an array or object can you access with {{custom_arg.product_name}}, etc., so I can't comment on the product object you passed over

Just set it up and test it and you can see if the data comes across in the logs / queue.