How do you add users to a database collection after paying for membership through PayPal?

I’m wondering how are you supposed to add users to a database collection after they have paid for their membership on PayPal. I know this would be easy to do with Wix paid plans, but that’s not an option for me.
I’ve been following a tutorial which you can find by searching: Add PayPal To Your Wix Site Using Velo & PayPal REST API by Dude Lemon (sorry this forum doesn’t let me embed links) anyways after following the tutorial, I don’t know how to add a wix web hook to listen for a payment confirmation from PayPal.

This looks like what you need. Then query users for their email, then give the match a role that confirms they have payed.