Login from subdomain

I am wondering if there is a way I can login to contao from a subdomain?
So subdomain.domain.com has a login page, once I'm logged into that page, I can visit www.domain.com and be logged in.

Thanks

Do you mean a front end login? There is a Contao 3 extension for that: https://github.com/richardhj/contao-crossdomaincookies

You could ask the developer to make it Contao 4 ready.

Yes, front end login. Hmmm... That extension seems interesting. Though I might have come up with a solution, I don't know if it's the best or not.
I was thinking of creating an iFrame to a page that is only a login form. Size it right and all and then it should look a part of the original page.
Once they are logged in, it redirects to a page like I have it now, sets the session variables and then reloads the parents page and since they are now logged in, it takes them to their content and they are able to browse the entire site.
Not sure if this is good practice or not though?

What I'm doing is a non-contao page to a contao page.