It gave me hard time to trigger build from the Slack even if we created the Incoming - Outgoing webhooks in the slack. Build Triggers description is completely found to call the build from the API or trigger URL directly.
If you are facing the same problem by putting the URL on the given description, you need to change the Url like "JENKINS_URL:port/buildByToken/build?job=JOBNAME&token=Authentication_Token" replace JOBNAME and Authentication_Token according to your build.
