update: if the above link is not found, you can google search for "dropbear ssh ubuntu luks encryption" for the steps
After following the above steps, I create an entry called "unlock" in my ~/.ssh/config file on my Mac laptop. So after I boot my Ubuntu 16.04 LTS Desktop VM (using the VirtualBox CLI), then I type 'ssh unlock', and type 'unlock' when presented with the busybox prompt. I am prompted to enter a password to unlock the encrypted disk. After that, the system will boot up, then I can ssh in remotely.
Host unlock Hostname 192.168.56.101 User root IdentityFile /Users/ksa/.ssh/id_rsa_ubuntu_unlock StrictHostKeyChecking no