Remap function key on external logitech keyboard
It is possible to do with the Karabiner-Elements app.
As you have completed the installation, head to the app. In the very first tab choose the input device you wish to make a key remap for, then add a new item and define the initial key and target key (they are in the top section of the drop down).
This is it, no need to reboot. It effects immediately.
PS. it also allows you to remap the Eject key, which becomes useless if DVD drive is taken out for an extra HDD/SSD.