I've tried a number of options, none of them worked:
notify-send 'text'
kdialog --passivepopup 'text' 5
qdbus org.kde.knotify /Notify event "event" 'app' "(" ")" 'title' 'text' 'pixmap' '' 5 0
Update: first two options should work, in my case the problem is a KDE bug with autohiding panel: KDE5 popup notifications not shown if panel is autohidden