Using environment variable on logstash.yml

Good afternoon everyone
I'm trying to follow the documentation example - https://www.elastic.co/guide/en/logstash/6.5/logstash-settings-file.html - referring to the use of variables in logstash.yml more specifically in the node.name attribute I'm putting it as in the example but it does not work:

Here is the excerpt from my logstash.yml that I use:

## ------------  Node identity ------------
#
## Use a descriptive name for the node:
#
node.name: 123_${HOSTNAME}
#
## If omitted the node name will default to the machine's host name

##--------------------------------------------

My logstash is in version 6.5.4, has anyone else gone through this?

Obs: No have nothing in log.

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