How to pass a variable to another domain using js?

Question

JavaScript

There are two sites with vanilla JS
Site1 links to Site2.
it is necessary to transfer the referrer from Site1 to Site2 (that is, the one from where the user got to Site1)
cookies work within the same domain;
you cannot conditionally add parameters to location.href;
the option of loading site1 in an iframe on site2 with cookie check certainly works, but it's too cumbersome to load.
How else can you pass a value?

Answer 1

Not sure, but maybe localstorage?

Answer 2

Make a common api.domain.tld, take CORS-Ajax with withCredentials from anywhere, session as standard, in cookies?