Everything I have tried either doesn't work or results in an error while starting the service.
This is what chapt GPT suggested and it resulted in an error...
ExecStartPre=/bin/bash -c 'echo "Your message" >/dev/stdout'
The error I saw was:
"/bin/bash: /dev/stdout: No such device or address"
In short, you don't. Services do not produce interactive output and are not attached to the original 'systemctl' terminal in any way – they're services, and their output goes to the system log (journal). Anything you 'echo' will be visible in systemctl status of the service.
(Second, redirecting something to > /dev/stdout is completely redundant, as the text from 'echo' is already being written to stdout, so you're just redirecting stdout to itself. It's just that the stdout of a service is not attached to your terminal – it's attached to a log pipe.
The redirection fails with an error message because of the strange way /dev/fd works on Linux; it's a bug that went unfixed for two decades and will never be fixed. Services do have an "stdout", but it is attached to an anonymous pipe, not a file, and cannot be re-opened the way /dev/fd tries to do it.)
wall, assuming your terminal emulator opts in (i.e. if the terminal windows show up in who or w). Otherwise, make something in ExecStartPre that sends you an email or a chat message. (I have syslog-ng set up to forward "alert"-level syslog messages to a Discord webhook, so my tools can do logger -p alert ... and it results in a push notification.)