$ cat /etc/salt/master.d/reactor.conf:

reactor:
  - 'salt/job/*/new':
    - '/srv/salt/reactor/highstate_any.sls'


==================================================================
$ cat reactor/highstate_any.sls:

{% if data['fun'] == 'state.highstate' %}
highstate_any:
  runner.state.sls:
    - mods: slack_highstate
{% endif %}

==================================================================
$ cat state/env/base/slack_highstate.sls:

slack_highstate:
  slack.post_message:
    - channel: '#salt'
    - from_name: Salt
    - message: 'Highstate execution activated...'
    - api_key: xoxp-xxxxx-blablabla