Redirecting after checkout

[FerAA]

Hi! So I have an independent web page that has a Buy Button that redirects you to Shopify's checkout. After the checkout I redirect back to my web page (with additional script) but I also want to create a tracking variable during the checkout so I know in fact that the user went through the checkout and not just typed the url. I tried using localStorage.setItem in the additional script but it didn't work.

Thank you!