Howto write a message to syslog from lua?

I would like to write a message to syslog file when a backup is generated.
How can I do it.
I tried to include
ubus call log write '{"event":"SYSBACKUP: A new system backup was generated and downloaded to your computer."}'
in /usr/lib/lua/luci/model/cbi/admin_system/backupfiles.lua but this aparently ignored (nothing was written to syslog.
I tried to put this line as first executable line of the script, with no success too.
What I'm doing wrong?

Now I tried
conn:call("log", "write", { "event":"HSMN: Mensagem gravada via ubus" })
Don't works too