Manipulation Assertion Consumer Service URL

Hello!

I have an IdP configuration working 100% perfectly in conjunction to a third party SP. The issue I am getting is the following: The SP have custom links that are provided to users. When the users click on this links on a new session/new browser, the SP will do what is told to do, take the request to the IdP for authentication and if successful, will "redirect" it to the SP with the SAML auth. BUT the user is landing in the default page instead of the custom link.

Example:

User -> Click on Default SP Page -> IdP -> SAML -> SP - Default Page
User -> Click on Custom URL (different from Default) -> IdP -> SAML -> SP Default page

So, how I can manipulate the Assertion Consumer Service URL and/or preserve the RelayState gather in the SAML?

Thank you

J