The URL for the confirmation page or Thank you page that a user will land on once a purchase has gone through will start with mysite.thinkific.com/orders/thankyou/*
The * in the above example will be a unique variable that is created for each order that is processed successfully, so there isn't a specific page under mysite.thinkific.com/orders/thankyou.
If you are setting up a tracking goal on Google Analytics, you can use the option starts with or contains so that Google will track any URL that contains your Thank you page.
If you're using our Meta Pixel integration, we already track a "purchase" event on the Thank you page for all your products. Find the full details here.
Another option would be to use the URL of the first lesson of your course. Typically, this is the next page that students visit after purchasing the course.