Actually, you add the nextcloud account either in Kaddressbook or in Korganizer/Kalender, or in Akonadi konsole. There was something called “online accounts” in Plasma but I never understood what it was good for, mostly obsolete instant messaging services, but not the integration of Nextcloud. KDE/Plasma has nowhere near the consistency that GNOME has. Sure, it generally offers more functions and options, but it is certainly more messy.
Actually, you add the nextcloud account either in Kaddressbook or in Korganizer/Kalender, or in Akonadi konsole. There was something called “online accounts” in Plasma but I never understood what it was good for, mostly obsolete instant messaging services, but not the integration of Nextcloud. KDE/Plasma has nowhere near the consistency that GNOME has. Sure, it generally offers more functions and options, but it is certainly more messy.