How to check if a repost was made from the site?

Question

PHP

We plan to add a function to the site "Make a repost and get a discount coupon". In order to implement this, we need to understand whether or not the user actually reposted from our site.
Scenario: A user came to our site. By default, all users are anonymous (unauthorized). Then he was offered to repost and get a discount coupon. He made a repost, we started checking (what kind of question?) And if there was a repost, we displayed a promotional code for a discount.
Tell me,
1. How can I check if a site visitor made a repost or not?
2. How quickly is a repost made indexed in a social network search engine?
For ourselves, we distinguish two ways to implement the task:
1. For each repost, we form a unique link into which we sew the user ID. For example, the repost link will look like this: mydomain.com/?id=33. Then, through the social network API, we look for this link, and if it exists, we mark the presence of a repost. At the same time, there are fears that the repost indexing in the social network search will not be in the moment. In this case, we need to do several checks, and this increases the load on the server.
2. Before making a repost, the user logs in through the social network. In this case, we get its ID and access to the profile. As a result of the repost, we will be able to check using the received user token. But there is an authorization element here, which cuts the conversion.
We would be grateful for your advice on solving our problem.

Answer 1

Look towards Oauth
Anonymous authorizes on your site via Oauth to receive a promotional code
. You check the fact of the repost via the API (VK/OK/FB) and make a decision to issue a promotional code.
Regarding the authorization element that cuts the conversion: this is the issuance of a promotional code. If the visitor has already made a repost, the bounce rate promises to be low. Give the authorization form before the issuance of promo ...

Answer 2

to get a discount - a form with a link to the repost field.

Answer 3

See how the social lock works.

Answer 4

Indexing is clearly not instantaneous. If the user ID is known in advance, then you can check for a repost through the API. If not, then openAPI