Setting USB flash writable to particulla user in /etc/fstab

How can i set in /etc/fstab for a particular user to allow usb flash drive writable when mounted .

e.g. how to allow user fred in /etc/fstab to access usb flash drive writable when mounted

/dev/sda1  /mnt/usbflash  vfat defaults, users,_____?____=fred,umask=022,0,0

I'd get the 'defaults' out and try 'user' instead of 'users'. I don't believe it takes individual users, but if you mount it on it's mount point and run

chown -R  <user:group> <mountpoint>/*
chmod -R 0700 <mountpoint>/*

filling in the correct options you should have it so that the chosen user can do stuff and other users can't.

Try something similar to this:

/dev/sda1 /mnt/usbflash vfat rw,users,uid=500,gid=500,umask=022 0 0

Replace "500" with your uid, gid.

Let the user mount it?

/usr/bin/udisksctl mount -b /dev/sda1 /media/fred/quarry > /dev/null 2>&1

Anything after /media/fred/ is arbitrary and is not a fixed point. It does not have to "exist"
Fred will full permission. No chmod/chown nonsense.

You can specify permissions for the mount in fstab. But it's better to specify permissions in the file system.

Since it's vfat (fat32 or fat16), specifying file permissions in the file system isn't an option.