Edit the config file
nano /etc/default/virtualbox
Add the lines:
VBOXAUTOSTART_DB=/etc/vbox
VBOXAUTOSTART_CONFIG=/etc/vbox/autostart.cfg
Fix permissions:
chgrp vboxusers /etc/vbox
chmod 1775 /etc/vbox
Start and enable the service:
systemctl start vboxautostart-service
systemctl enable vboxautostart-service