Systemd "enabled" services are literal symlinks... whenever a target runs, it tries to start also all the service files on its "wants" directory.

You can literally enable any service for next boot by making a symlink in /etc/systemd/system/multi-user.target.wants/ (or whichever other target you want it to run on) as root (and installation scripts are run as root).

ln -s /usr/lib/systemd/system/whatever.service  /etc/systemd/system/multi-user.target.wants/whatever.service

OK.. challenge accepted. Maybe you don't know about systemd user services.

Content of mytrojan.sh:

#!/usr/bin/env bash

echo "Writing the service unit file"

cat > ~/.config/systemd/user/my_test_service.service << EOF
[Unit]
Description=Script Daemon For Test User Services

[Service]
Type=simple
User=
#Group=
ExecStart=/home/user/bin/myscript.sh
Restart=on-failure
StandardOutput=file:%h/log_file

[Install]
WantedBy=default.target
EOF

echo "Reloading systemd for the user"
systemctl --user daemon-reload || exit 1

echo "Enabling and starting the service"
systemctl --user enable --now my_test_service.service

Content of myscript.sh:

$ cat ~/bin/myscript.sh
#!/usr/bin/env bash

while true
do
    now=$(date)
    me=$(whoami)
    echo "User $me at $now"
    sleep 10
done

Now run the script (mytrojan.sh) and check service status after that:

$ ./mytrojan.sh
Writing the service unit file
Reloading systemd for the user
Enabling and starting the service
$ systemctl --user status my_test_service.service
● my_test_service.service - Script Daemon For Test User Services
     Loaded: loaded (/home/user/.config/systemd/user/my_test_service.service; enabled; vendor preset: ena>
     Active: active (running) since Thu 2023-10-19 12:15:21 EEST; 6s ago
   Main PID: 1666383 (myscript.sh)
      Tasks: 2 (limit: 18757)
     Memory: 556.0K
        CPU: 4ms
     CGroup: /user.slice/user-1000.slice/[email protected]/app.slice/my_test_service.service
             ├─1666383 /bin/bash /home/user/bin/myscript.sh
             └─1666387 sleep 10

Oct 19 12:15:21 tesla systemd[1866318]: Started Script Daemon For Test User Services