Create /etc/rc.local, containing:

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

su -l slack -c "VBoxManage startvm alito --type=headless"
exit 0

Change permissions:

chmod +x /etc/rc.local

Activate it by adding the following content to /etc/systemd/system/rc-local.service:

[Unit]
Description=/etc/rc.local
ConditionPathExists=/etc/rc.local

[Service]
Type=forking
ExecStart=/etc/rc.local start
TimeoutSec=0
StandardOutput=tty
RemainAfterExit=yes
SysVStartPriority=99

[Install]
WantedBy=multi-user.target

Enable the rc-local script on boot:

systemctl enable rc-local

Start the rc-local script:

systemctl start rc-local.service

Check if any error occurred while starting the service:

systemctl status rc-local.service