Thanks @lme for this!
add this line
That's it
Then insert a USB device, go to /media/msdosfs/$label and the device is mounted automatically. Some seconds after the last access to a file in /media/msdosfs/$label is made, the device gets unmounted.
Code:
cd /usr/ports/sysutils/automounter
make install clean
cd /etc
ee rc.conf
Code:
automounter_enable="YES"
That's it

Then insert a USB device, go to /media/msdosfs/$label and the device is mounted automatically. Some seconds after the last access to a file in /media/msdosfs/$label is made, the device gets unmounted.