Tags: [dunst][dunstctl][notification]
The Arch wiki article says it all.
With notify-send (works with all notification daemons):
notify-send 'Test notification'
notify-send -u low|normal|critical 'Notification with urgency'
With dunstify (dunst tool working only with dunst):
dunstify -h string:x-dunst-stack-tag:test Test -A 'tested,default'
dunstify -h string:x-dunst-stack-tag:test Testing