Using a multibranch pipeline to poll pull requests from Azure DevOps

Our corporate policy does not allow inbound traffic so rather than using webhooks I am trying to poll. Both the basic pipeline and the multi-pipeline poll to find the Jenkins file in the branches. But it does not find the jenkinsifile which resides in a pull request branch. Before I complete the pr I want to validate the pr using the Jenkins build, but the build does not find it. I have tried different refspecs and I tried to find extensions for Azure DevOps to no avail. Just don’t know why it is hung up.

Update: Jenkins : Azure DevOps and Team Foundation Server Plugin state the plugin is no longer available.