Splunk plugin - Event enrichment

I have the Splunk plugin correctly configured and events are sent to the Splunk.

I would like to send a extra field (Jenkins env. var.) in the result event after each pipeline build.

How to proceed ? I have tried to use “Customize Event Processing Command” without any success. See:

https://community.splunk.com/t5/Getting-Data-In/How-to-send-few-values-from-jenkins-pipeline-to-splunk-and/m-p/384470
https://wiki.jenkins-ci.org/display/JENKINS/Splunk-Plugin-for-Jenkins.html

I found it:

To activate the “Customize Event Processing Command”, you have to activate the data sources “Build Event” and “Build Report” too.

Here my code for who would like to use this plugin (more detailed than the current documentation)
JENKINS-70086