Automatic sign-up between different domains without cookies?

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I'm trying to find a website that does the following:
1. Signs you up to a second site under a different primary domain (i.e.
as a result of a checkout or something). The username and password may
be automatically generated.
2. Logins for you and redirects you to the second site without the need
of you signing up or signing in

I can't find an example of such a site. Do you know any?


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