multibranchPipelineJob(…) {
    branchSources {
        branchSource {
            source {
                github {
                    id(…)
                    credentialsId(…)
                    repoOwner(…)
                    repository(…)

                    // unfortunately these are mandatory:
                    configuredByUrl(false)
                    repositoryUrl('')

                    traits {
                        gitHubBranchDiscovery {
                            strategyId(1)
                        }
                        gitHubPullRequestDiscovery {
                            strategyId(2)
                        }
                        githubTagDiscovery()
                    }
                }
            }
            strategy {
                defaultBranchPropertyStrategy {
                }
            }
        }
    }
}