ping.sls:

{#

This variable will be buried inside the {{ data }} dictionary. So I expect the following conditional will render to an empty string. Also it's 'post' not 'postdata'. Try {% if data.post.secretkey ... %} instead.

#}
{% if postdata.secretkey == "<secret>" %}

{#

The sls data structure in Reactor files is different than regular sls files because it's being run on the master and in a different context. Try this instead:

pingtest:
  cmd.cmd.script:
    - tgt: <sometarget>
    - kwarg:
        user: root
        group: root

#}
pingtest:
  cmd:
   - script
   - name: salt://netapi/test.sh
   - user: root
   - group: root

{% endif %}

pingscript.sls

pingtest:
  cmd:
   - script
   - name: salt://netapi/test.sh
   - user: root
   - group: root

Invocation:
curl -H "Accept: application/json" -d tgt='base-lj-3' -d secretkey="<secret>" -k https://<ip>:8888/hook/ping