There is no ftp service listening on the Antminers (at least not on my L3+).
You can use putty to login as root and add whatever you want to /etc/hosts.
Take note that there is also no nano installed, so your command won't work, even if you had sudo installed.
Also I believe that upgrading the firmware should get rid of that "Feature". Elsewise if you don't want to upgrade firmware or just want to make sure in case, you can add the line with the following command to your hosts-file when logged in as root via putty:
echo "127.0.0.1 auth.minerlink.com" >> /etc/hosts
HTH