How to detect the transition of users from instagram to vkontakte?

Question

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Task: understand how many inst subscribers converted into vk subscribers. In the active instagram link there is a link to the VKontakte group. I can't figure out how to catch transitions without utm tags and shorteners. Give me ideas
P.S.: In statistics, most likely, clicks are recorded as clicks on a direct link.

Answer 1

Make an intermediate pass or skip through the reducer with statistics, there is no other way. Although mb VK can utm tags ... dig in this direction