If you have your own DNS server you can set a hostname there like ‘jellyfin.myserver’ and have that accessible from your internal network. If you want to do so on your PC you can edit your hosts file to add a custom entry. https://www.hostinger.com/tutorials/how-to-edit-hosts-file
Here is the link to the thread, on internet archive because fuck spez
https://web.archive.org/web/20230618012147/https://old.reddit.com/r/Music/comments/14c5kyw/update_bizarre_popup_admin_account_demands/