sudo nano /bin/connect-bluetooth.sh
In that file add
#!/bin/bash
sudo bluetoothctl << EOF
power on
connect MAC Address »
exit
EOF
Save that file.
Make it executable
sudo chmod +x /bin/connect-bluetooth.sh
Then create a new file
sudo nano /etc/systemd/system/connect-bluetooth.service
Add this text:
Unit »
Description=Connect Bluetooth
Service »
Type=oneshot
ExecStart=/bin/connect-bluetooth.sh
Install »
WantedBy=multi-user.target
Save that file.
Then run this command to enable that process
sudo systemctl enable /etc/systemd/system/connect-bluetooth.service