Custom login.html logic for SSO

Howdy peeps   I have a scenario where I need to have custom logic in my login.html which forwards me to a request handler for SSO. At the moment that URL is hardcoded, which is obviously not the best. Is there a way I can set the value in a constant and pull it through to make deployments much easier?   Thanks
1 answers

In the redirect supply the callback url as a parameter, although in other cases this is setup in the config of the IDP and is used based  on the incoming SP