command:

systemctl hibernate

I created my swap partition like this: https://links.lburgy.ch/shaare/WSOAMQ

On Kubuntu 24.04, create the file /etc/polkit-1/rules.d/10-enable-hibernate.rules with this content:

polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.login1.hibernate" ||
        action.id == "org.freedesktop.login1.hibernate-multiple-sessions" ||
        action.id == "org.freedesktop.upower.hibernate" ||
        action.id == "org.freedesktop.login1.handle-hibernate-key" ||
        action.id == "org.freedesktop.login1.hibernate-ignore-inhibit")
    {
        return polkit.Result.YES;
    }
});

https://www.reddit.com/r/Kubuntu/comments/1c2frkd/enabling_hibernation_on_2404/