Why can't I get a callback from a custom VK share button?

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There is a custom "Share" button on the page, cobbled together from css. Sharing URL is suspended on it.
Sharing is happening correctly, the task was supplemented by tracking successful sharing.
The docs nicely describe how to implement such callbacks through the button widget.
But it does not suit me - I have a custom button.
And when I click on my button, this code doesn't work.
VK.Observer.subscribe("widgets.like.shared", ...
The case is the most common in my opinion, but the information on the Internet just cried like a cat ...