Slack Notifications as shown below is not available directly in the Job configuration but it is in Post-build Actions

These options are working correctly in Post-build Actions but this is a bit misleading that post-build action specifies something which should be before build.

However, plugin v1.8.1 installed on Jenkins 1.625.3 has the expected options as shown in the https://wiki.jenkins-ci.org/display/JENKINS/Slack+Plugin