Yeah, changing the default STS in Sitefinity was a false start as it will never create the user in Sitefinity after authenticating with the custom STS.
If you take a look at the default login page that Sitefinity creates you'll see the link to your custom STS if something like this
I'm guessing Sitefinity stores the correct page to return to in the database against that Guid.
I am currently working on the configuration on Sitefinity application with External Authentication based on the posts above and the steps mentioned in the link docs.sitefinity.com/for-developers-configure-custom-external-openid-connect-provider.
For my application, the Identity server application user is authenticated successfully in the external STS application but it redirects me back to the Homepage of my Sitefinity application (which is my login page). Even if I try to access urls of other pages in CMS, the "Redirect URi" doestn't work.
Can anybody suggest as how can i Modify the "Redirect URi" so that i can get the Sitefinity application redirected to the page i precisely want to ?