jQuery withCredentials not working in Safari?
I'm sorry to say, but there isn't a very elegant solution for this problem.Apple had recently adopted a strict policy to prevent 3rd party cookies - link
This means users only have long-term persistent cookies and website data from the sites they actually interact with and tracking data is removed proactively as they browse the web.
The best solution I can think of is to redirect the user to a login page hosted in the 3rd party domain and then back to the original page after the login. This way the cookie will be stored (since the user interacted with its domain directly), and you would also be able to interact with it as 3rd party cookie from the original domain.