Your Spreadshop comes with a Google Analytics and a Facebook Pixel connection by default.
By using Google Analytics you will be able to keep track of your visitors and even more, watch E-commerce transactions and Ad conversions.
Sign up for a Google Analytics account, make sure to create a Universal Property and connect it with your Shop.
Afterwards you need to connect your Google accounts (Ads + Analytics) to ensure data streams into both directions.
Another Tip:
Your question regards a success page, indicates you are playing around with the Conversion Goals feature. You don´t have to set that up, as your shop delivers all e-commerce tracking related data, when a sales was made, within the in-built E-commerce tracking.
Just make sure it is enabled within Google Analytics.
If you are not familiar with the differences of E-Commerce Tracking and Conversion Goal tracking, please make sure to read this in advance:
Goals in Google Analytics help you keep track of the actions your website visitors did or didn’t take. A goal conversion (or goal completion) takes place when your visitors complete a specific action you are tracking. Like downloading a PDF, visiting a page that is important to you, playing a certain video.
Ecommerce tracking is a feature of Google Analytics through which you can track ecommerce data (like sales amount, number of orders, billing location, average order value, etc)