Subdomain redirect puzzle

[ChroniC]

Ok, might not be a puzzle for someone who knows what they're doing but atm without taking down a live site and testing my theory, I need a bit of help getting my head around if this is possible.

So i have been asking by a client, to do the following (if possible)

Website A (on a custom built Laravel CMS with a client login to a user area)
Website B (new Wordpress setup)

The customer wants a new website but doesn't want to pay to move the functionality of the customer area. So what I need to do is have a login menu option which goes to the old server login page.
I am hoping this can be done something like this.

Website B on my hosting with the DNS pointing to my hosting, have a subdomain of login.example.com pointing to the Website A old server where the Htaccess for that server redirects / to /login.
Hardcode all login buttons on Website B to go to the subdomain.

Would that work?
Thanks

 

[ChroniC]

Would subdomain.domain.com/login be enough to access the login page though, the problem my brain is having is that surely the site_url or home_url for the old site would have to be changed to the same as the subdomain for it to understand the link.
For example i cant just create a subdomain on my dns and point it too the BBC website ip address and expect it to work.