Not sure that’s useful, because it won’t work either way. If the SSO address isn’t valid externally then the browser won’t be able to reach it across the public internet.
SAML is an authentication protocol, not an IdM or IdP - it has to consume data from somewhere else. It may be that your IdM also supports SAML, but unless you can tell us what this backend system is we’re going to struggle to help you here.