Source: https://www.howtoforge.com/tutorial/passwordless-encryption-of-linux-root-partition/
The process of entering the passphrase at boot time will now be automated using an USB memory stick. Instead of using a passphrase , the secret key on the USB will decrypt the encrypted volumes. Connect an USB stick to the VM and locate it using the dmesg command. It is detected as /dev/sdb in my VM.
The secret key of 8192 random byte is extracted from the usb stick using the dd command.
dd if=/dev/sdb of=/root/secret.key bs=512 skip=4 count=16