Redirect user sign-up from a specific page to another specific page

Hi,

I want to create a single page, not “linked” with my main site, a “mini-site” which works with my main site features.
So i change the menu and links etc… and i create a subdomaine for it.
I want the user stay on this mini-site and don’t be redirect to my main site.
My problem is : When a user sign-in or sign-up on my mini-site (sd.mysite.com), he’s redirected to main/root page (mysite.com).
There is a solution to have a specific redirect after login or sign-up for a specific page ?

Thanks for helping and sorry for my english, i’m french.
Charles