How to implement a window with a suggestion to enter the site using the details of the google account?

Question

Google Chrome

I visited one site where a window appeared in the corner, in which there was an offer to enter the site using the details of my google account.
I was wondering how to implement this in my project? Searches brought me to https://developers.google.com/identity/one-tap/web/
But there is a warning text

The beta test program for this API is currently closed. We are improving the API's cross-browser functionality and will provide updates here in the coming months.

But if this API is closed, then how does the site on which I saw this window use it? Thank you all in advance.
UPD.
site https://megabonus.com/

Answer 1

Or maybe the site itself?