Unable to write a network configuration file on the flashed sd card

I would like to add, on linux (Ubuntu) a configuration file (my-network) to the hassos file system in my just flashed SD card, with the credentials (SSID and PASSWORD) of my WIFI network as explained in the documentation.
However, after flashing the micro sd card, the resulting file system is read only. I even tried to remount it in read-write mode, but with no luck.
What can I do in order to write this configuration so it can be read at boot time?

the network info goes on a USB thumb drive, not the SD card. Re-read the docs