It appears that the notification plugin doesnt have the functionality to send a notification from a pipeline or a Jenkinsfile via a groovy command. That's pretty important especially when Jenkinsfiles overwrites configuration data for jenkins jobs, basically making it impossible to send notifications for any jobs (with this plugin) to a http endpoint.

Ideal functionality would be something like being able to write something like this in a pipeline:

pipeline {
  agent any
  post {
    always {
      notify( endpoint: "myUrl", message: "optional: job completed" )
    }
  }
}