Back when I was creating my original pool, I decided to use password prompt as my encryption key unlocking method. And it was good. But then I wanted to automate this a bit. I wanted my key to be read of USB drive.
To do that one can simply prepare a new key and point the pool toward it.
dd if=/dev/urandom of=/usb/key.dat bs=32 count=1
zfs change-key -o keylocation=file:///usb/key.dat -o keyformat=raw Pool
Of course, it's easy to return it back to password prompt too:
zfs change-key -o keylocation=prompt -o keyformat=passphrase Pool