save a particular sso website offline

I've bought a book, extra content are located on a web site with an etext platform that has to be activate with a code provided by the book. I've done this, but the material is available only until 2018, so I wish to download this material offline to study it and to save it for future. I have tried download this site with httrack also using the proxy to save the cookie for the access, being that is sso it continues to download files like login23yd and doesn't download the site, do you have some suggestions on how can I do? Thank you