if you have /etc/kernel/cmdline already, just add your switch there, and it will be honoured for future kernel installs. if you don#t then do cat /proc/cmdline > /etc/kernel/cmdline and then add it there.
EDIT:
That command only works in root, you need to alter it to work with sudo.
From my understanding, by default, Manjaro doesn’t use systemd-boot and the kernel is not installed via systemd (but pacman and hooks for mkinitcpio).
So, modifying a file /etc/kernel/cmdline will probably not work out-of-the-box.
The goal of this script is to generate the kernel image (at least depmod is run, and its pre-configured with a few hooks), so I guess it’s a replacement for mkinitcpio. And then copy it to its final destination at /boot/... somewhere.
In Archland, this is already done with mkinitcpio and you have to generate the systemd-boot loader files yourself.
I guess it’s possible to remove mkinitcpio and use a custom pacman-hook for this script but in the end it will probably take more maintenance time that simply copying the /boot/loader/entries for new kernel versions and adjusting your cmdline manually.