title: Get pull request Id from azure devop classic deployment pipeline
body: <p>I checked several sources, seems possible, but can't get it work.</p>
  <p>What I want is that whenever a PR is created and PR build is successful, then a PR release is run and a site is created (either with PR Id or PR source branch, e.g. pr122--xxx.azuresite.net).</p>
  <p>From an answer for <a href="https://stackoverflow.com/questions/61820405/get-the-pull-request-id-of-a-continuous-deployment-release-azure-pipelines">this question</a>,
  Looks like I could use release.artifacts.alias.PULLREQUEST.ID, but when I tried it, this doesn't work at all. Also from the official doc, there is no mention of this variable. Is this only for the new YAML pipeline?</p>
  <p>As most of our things are in classic pipeline, the solution I can think of is to parse the build source, which is refs/pull/11201/merge. Looks like 11201 is the Pull Request ID, so I just need to replace refs, pull,merge and &quot;/&quot;. But I am not sure if better approaches available.</p>