/etc/systemd/system/porridged.service:

[Unit]
Description=Porridge Daemon
Wants=network-online.target
After=network.target network-online.target local-fs.target

[Service]
User=slack
Group=slack
WorkingDirectory=/home/slack/porridge/daemon
Restart=always
ExecStart=/home/slack/.rvm/wrappers/ruby-3.0.0/ruby /home/slack/porridge/daemon/bin/porridged

[Install]
WantedBy=network-online.target

Then

systemctl enable /home/slack/appname/appname.service
service appname start

Check it is running with

service appname status

If it has failed to run, start looking in /var/log/syslog.