How to check the information about the user who entered the page?

Question

In contact with

I'll clarify right away, I'm new to this business and perhaps the task is trivial. Actually the task:
- It is planned to send out to potential customers with an offer to join the Vkontakte group and get a discount. The group is open (if it matters).
This is how I present the problem. The letter contains a link to the group and a link where you can get a discount coupon (our page), and after the user has joined the group, you can somehow check whether he has joined using the VK Open API. Has anyone solved a similar problem? how is it done in the mind?
Not necessarily on the NodeJS backend, but highly desirable, because I want to develop in it. and if you can do it without a backend at all, it will be even better.

Answer 1

1) Save the user's id in BD before mailing. And generate a UUID (or anything unique).
2) Generate a letter with a link like www.site.ru/sale/< uuid >
3) When the user follows this link, we force him to log in (if you have an application, install it).
4) Having received the user ID from VK, we make a request to #isMember
5) If such a user exists in the database, then we change his status (in the database, to `received` or whatever), and set the UUID key to the status `activated` (which -would not rummage this link).
6) We send/show the discount/key to the mail/VK/on the page