How to include a custom Slack message only on completion? We're running Jenkins with the Slack Notification Plugin. Is there a way to send the custom message only on completion, not on start? To be clear, I don't want to disable the build started message, I'd just like the custom text to only show on completion (success). 

Use case is jobs like delete stack. We use the custom message to make it clear which stack the job deleted, but you end up with slack messages like (paraphrased)

Delete job started
  Deleted Alex stack

Delete job finished
  Deleted Alex stack


Obviously you can see where this causes confusion. I'd rather get

Delete job started

Delete job finished
  Deleted Alex stack


"It can't be done" is an acceptable answer if that's the case. I know I can do a pipelines workaround but I'd love a quick fix for the jobs we haven't converted yet.
As far as I was able to tell it's not possible.

We solved this by using Pipeline putting the custom messages in the Jenkinsfile.

The flexibility gained by having the Jenkinsfile in the git repo really helped our developers have a better understanding of the build process, and allowed them to make necessary changes to the build commands without the communications headache.

It's a nice excuse to move over to Pipeline.

It also happens to be very easy to do:

slackSend (color: colorCode, message: customMessage)