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