Non Administrator cannot configure credentials in the Slack Notifier step.

It is possible to specify credentials in the Slack Notifier post build step however only an administrator can configure it:

In the screenshot above, as a non administrator I can actually create credentials from here but I cannot use them.

Reference: https://github.com/jenkinsci/slack-plugin/blob/slack-2.3/src/main/java/jenkins/plugins/slack/SlackNotifier.java#L398