How to change tty login prompt?

My background is RHEL-derived distributions (mainly Fedora, today), Arch is foreign to me.

Back when /etc/inittab and mgetty where used by RHEL, you could edit /etc/inittab and pass the -p option to mgetty. This would set the prompt.

Now with the adoption of systemd /etc/inittab is empty. The tty configuration file is now /lib/systemd/system/getty\@.service. This file seems to use agetty instead of mgetty to manage the tty. agetty man page doesn't show the -p option, but you could reinstall mgetty and use it. Or you could use the -n (no prompt) option with the -l (specify a login program) option and write a wrapper to /bin/login with your own prompt.

Now, I don't know how to set this up on Arch, but you have to find out where it manages its ttys (/etc/inittab is the first place you have to look), then see with the tty program how to roll your own custom prompt.