steps to do:

#goto default directory
cd /etc/default

#make a backup of grub config
cp -p grub

#edit file /etc/default/grub >> change there the GRUB_CMDLINE_LINUX_DEFAULT to:
GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0 console=tty0"

#aktivate the settings by running the command:
update-grub . /boot/grub/grub.cfg

>> now reboot and connect using a serial cable and a tool like Tera Term VT or a putty serial connection, make sure to use only 9600bps as setting.
>> enjoy this new feature, you can now administrate the machien without a display ;-)

tested on a debian based proxmox linux system:
pve-manager/8.0.4/d258a813cfa6b390 (running kernel: 6.2.16-3-pve)

see also:

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