This is specifically useful for Pi Zero W but it works for any Pi with onboard wifi.
Assuming you already have downloaded a Manjaro ARM image (any flavor) and have the urge to connect over WiFi to finalize the setup process.
Open a terminal and do
su -l root
Mount the root image from the sdcard
mount /dev/mmcblk0p2 tmproot
Create a network definition
cat << EOF >> root/etc/systemd/network/wlan0.network
Create a wpa_supplicant configuration - replace the variables $SSID and $PASS with your actual values
wpa_passphrase "$SSID" "$PASS" > tmproot/etc/wpa_supplicant/wpa_supplicant-wlan0.conf
Create a symlink to start wpa_supplicant for the wlan0
ln -s /usr/lib/systemd/system/wpa_supplicant@.service \
Unmount the partition and exit root context
rm -r tmproot
Insert the sd-card in your pi and power up
Cross posted at Manjaro Forum