How to link Azure DevOps pipeline with GitHub repo

Hi, I'm trying to configure a workflow on a GitHub project using the `Azure/pipeline@v1` action and facing a problem. I can successfully trigger the pipeline in Azure DevOps, but it always checkout the `master` branch from github rather than the PR opened.
I can see the log is trigger from the following: https://github.com/Azure/pipelines/blob/master/src/pipeline.runner.ts#L82
but it is unclear how I can do the link suggested by the comment on line 76
> // If definition is linked to existing github repo, pass github source branch and source version to build.

My pipeline on DevOps uses multiple checkout (self and a github repo using a GitHubServiceConnection) and is hosted in a DevOps repository.

Thanks for you help.