cd /etc/nginx/sites-enabled
sudo ln -s ../sites-available/<sitename>
# to disable the site just remove the symlink
sudo rm /etc/nginx/sites-enabled/<sitename>

# creates a symbolic link, when LINK_NAME omitted the link uses the name from the TARGET
# ln -s TARGET [LINK_NAME]

sudo nginx -t
sudo nginx -s reload

# via systemd
sudo systemctl reload nginx 
# or full restart
sudo systemctl restart nginx