How do I load a Jenkins Shared Library in a Jenkins Job DSL seed?

There are two option

1. Using jenkins global shared lib setting in gui see documation https://www.jenkins.io/doc/book/pipeline/shared-libraries/

2. Importing shared library inside jenkins file similart like this follow this blog https://tomd.xyz/jenkins-shared-library/

   library identifier: 'mylibraryname@master',
        //'master' refers to a valid git-ref
        //'mylibraryname' can be any name you like
        retriever: modernSCM([
          $class: 'GitSCMSource',
          credentialsId: 'your-credentials-id',
          remote: 'https://git.yourcompany.com/yourrepo/private-library.git'
    ])
   
    pipeline {
        agent any
        stages {
            stage('Demo') {
                steps {
                    echo 'Hello world'
                    yourCustomStep 'your_arg'
                }
            }
        }
   

P.S. When using "@Library _" the this means importing all libraries

It's possible to use jobDSL from Pipeline. (Im using multibranch pipeline as it allows to configure pipelineTriggers) You can configure your seed job to be a pipeline like this:

def gitCredentialsId      = 'github-jenkins'
def jobsRepoName          = 'https://github.com/my-jobs-repo.git'
def sharedLibraryRepoName = 'https://github.com/shared-library-repo.git'

properties([    
   pipelineTriggers([githubPush()])
])

pipeline {
    agent any
    stages{
        stage('Seed Job') {
            agent any
            steps {

                checkout([
                    $class: 'GitSCM', 
                    branches: [[name: '*/main']], 
                    doGenerateSubmoduleConfigurations: false,
                    extensions: [[$class: 'RelativeTargetDirectory', relativeTargetDir: 'shared-library']], 
                    submoduleCfg: [],
                    userRemoteConfigs: [[credentialsId:  gitCredentialsId, url: sharedLibraryRepoName ]]
                    ])

                git url: jobsRepoName, changelog: false, credentialsId: gitCredentialsId, poll: false, branch: 'main'
                jobDsl targets: 'jobs/**/*_job.groovy', additionalClasspath: 'shared-library/src'
            }
        }
    }
}

This way we can configure our seed job to checkout shared libraries repo into workspace subdirectory shared-library and specify additionalClasspath for jobDSL groovy scripts

So in your groovy scripts you can simply use import without @Library annotation

Setup pipeline name and default version (e.g. master) in "Global Pipeline Libraries" settings. Then use them in your Jenkinsfile as following

#!/usr/bin/env groovy
@Library('YourGlobalPipelineName@YourDefaultVersion') _

pipeline {

}