What is the proper path to the RemoteAuth end point if we are using a custom domain (CNAME)?

We've tried a wide range of paths and always get a 404.  Examples include:

https://support.yourcompany.com/support/RemoteAuth

https://support.yourcompany.com/RemoteAuth

https://support.yourcompany.com/portal/RemoteAuth

https://support.yourcompany.com/portal/home/RemoteAuth

https://support.yourcompany.com/portal/home/RemoteAuth

https://support.yourcompany.com/support/portalname/RemoteAuth

https://support.yourcompany.com/portalname/RemoteAuth

What are we doing wrong?