Hi there @banagan76!
Don here from Shopify.
Thanks for checking in with us here on our Community forums!
As the checkout page URL for orders made using Shopify is not static, you would instead just need to use the general formula for the thank you page.
Merchants wanting to track thank you page visits should use /checkout/thank_you.
You can add this to the end of your own store's domain name to get the full version or add this extension if that is what is required (as per Google Analytics).
It sounds like you're still getting set up with your current store, and working on your marketing requirements, is that correct?
Do you know what you need to work on next for this or would you have made up a marketing plan to help you organise your thinking?
All the best!