The git-plugin exposes a notifyCommit endpoint which can be used to trigger Jenkins jobs from external systems when commits are pushed to a git repository. This requires the pollSCM trigger to be setup on the job.

I can't find the ability to setup the poll SCM trigger on the organization folder. The option exists on the grandchild job (the workflow job of the multi-branch pipeline job) but is unselected and read only.

After spending time digging through code and documentation I found I can explicitly setup the trigger in the declarative Jenkinsfile pipeline:

triggers {
    pollSCM("")
}

This could be documented somewhere and perhaps added to the UI.