Linking cross-domains

[IcarusHU]

Hi,

we are trying to track users through cross-domains with linking.
Almost all of our pages uses GTM for this, which is working well, loading the GA with linker.
However we have a login part to a demo site.
We have GTM loaded in the login and the demo site too, so it's working automatically.
The only problem is we use OpenID connect, where the auth endpoint is a no-output URL which is authenticating and redirecting the users. For this, I've used your solution to send pageview event from PHP, however, because there is no GTM loaded here, and also couldn't add autolink with the proper domains, we lose the connection here.

Can I achieve this somehow?