I've faced the same issue and the dsl generated code does not seem to add the right config to the job config.xml.
For example, jenkins generated config (if i fiddle with the UI post-build action) generates this
```
<publishers>
<jenkins.plugins.slack.SlackNotifier plugin="slack@2.1">
<teamDomain>team-name</teamDomain>
<authToken></authToken>
<authTokenCredentialId>auth-token</authTokenCredentialId>
<room>#jenkins</room>
<startNotification>true</startNotification>
<notifySuccess>false</notifySuccess>
<notifyAborted>false</notifyAborted>
<notifyNotBuilt>false</notifyNotBuilt>
<notifyUnstable>false</notifyUnstable>
<notifyFailure>true</notifyFailure>
<notifyBackToNormal>false</notifyBackToNormal>
<notifyRepeatedFailure>false</notifyRepeatedFailure>
<includeTestSummary>false</includeTestSummary>
<commitInfoChoice>NONE</commitInfoChoice>
<includeCustomMessage>false</includeCustomMessage>
<customMessage></customMessage>
</jenkins.plugins.slack.SlackNotifier>
</publishers>
```
and the one from the dsl plugin
```
<publishers>
<jenkins.plugins.slack.SlackNotifier>
<teamDomain>team</teamDomain>
<authToken/>
<authTokenCredentialId>auth-token</authTokenCredentialId>
<room>#jenkins</room>
<startNotification>true</startNotification>
<notifySuccess>true</notifySuccess>
<notifyAborted>false</notifyAborted>
<notifyNotBuilt>false</notifyNotBuilt>
<notifyUnstable>false</notifyUnstable>
<notifyFailure>true</notifyFailure>
<notifyBackToNormal>false</notifyBackToNormal>
<notifyRepeatedFailure>false</notifyRepeatedFailure>
<includeTestSummary>false</includeTestSummary>
<commitInfoChoice>NONE</commitInfoChoice>
<includeCustomMessage>false</includeCustomMessage>
<customMessage/>
</jenkins.plugins.slack.SlackNotifier>
</publishers>
```
The first one works, the second one does not.