There is no easy way to enable this today for PR. It is behind the VSTS function that we want to talk about shortly.
The way to get it working today is to do something like: add a webhook to call its own service endpoint. Within your service endpoint, you could then call VSTS for the build queue, and the build will need a step to post success / rollback to GitHub.
At the endpoint of the user service, you need to make sure that the user is logged in as a member in the GitHub repository.
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