You can create a unit file for btsync (probably should be done automatically by the deb package).
Create the file: /etc/systemd/system/btsync.service
Containing:
[Unit]
Description=Bittorent Sync service
After=network.target
[Service]
Type=forking
User=btsync
Group=btsync
ExecStart=/usr/bin/btsync
Restart=on-abort
[Install]
WantedBy=multi-user.target
Once that file is in place enabling the service should work
sudo systemctl enable btsync