Declarative Jenkins pipeline which reads a shared library from a private Git repo

Jenkinsfile

/*
Firstly, you need to add a new Credential in Jenkins to access your Git.
Grab the ID of the Credential and use it in the credentialsId field below.
*/

library identifier: 'mylibraryname@master', 
    //'master' refers to a valid git-ref
    //'mylibraryname' can be any name
retriever: modernSCM([
  $class: 'GitSCMSource',
  credentialsId: 'your-credentials-id',
  remote: 'https://github.com/monodot/private-jenkins-library.git'
])

pipeline {
    agent any
    stages {
        stage('Demo') {
            steps {
                echo 'Hello world'
                sayHello 'Dave'
            }
        }
    }
}