This send a NTFY on bootup and shutdown

nano /etc/systemd/system/ntfy-notify.service

[Unit]
Description=Sends NTFY on bootup/shutdown
After=multi-user.target

[Service]
Type=oneshot
ExecStart=/usr/bin/curl -d "%H booted up" 'http://ntfy.sh/topic'
ExecStop=/usr/bin/curl -d "%H shutting down" 'http://ntfy.sh/topic'
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

systemctl daemon-reload

systemctl enable ntfy-notify.service