Hi Ann,
I was able to find the values of sonar.pullrequest.provider, sonar.pullrequest.vsts.instanceUrl, sonar.pullrequest.vsts.project and sonar.pullrequest.vsts.repository by logging them out in an Azure Pipelines run. After adding those properties to my CI config, I was able to run the build with PR decoration from my external CI. So my issue is now resolved 
Thanks,
Adam