systemd is very simple and easy to use, here you can see how to run the program in a screen with the specified user
create new file

nano /etc/systemd/system/radio.service

and put it into a new file

[Unit]
Description=Radio DJ
After=network.target

[Service]
Type=oneshot
ExecStart=/usr/bin/screen -dmS screennameradio /etc/liquidsoap/radio-pro.liq
ExecStop=/usr/bin/screen -S screennameradio -X quit
RemainAfterExit=yes
User=radiodj

[Install]
WantedBy=multi-user.target

and, of course

systemctl enable radio
systemctl start radio