A user is registering for a web app, they finish registering and get saved to a database.
I got a payment service signup link from a third-party payment processing service (GoCardless) that I can then redirect the user to, they fill out their bank details and get redirected back to the website.
This is to get away from having to code all this up in the app itself, just some redirects.
What I'm not so sure on is how to link the user that just signed up in the app to the user filling out the payment form, because they might not necessarily use the same email.
There must be some common process for this?