The Azure DevOps API article Evaluations - List describe how to receive a list of all the policy evaluation statuses for a specific pull request.
To call
GET https://dev.azure.com/{organization}/{project}/_apis/policy/evaluations?artifactId={artifactId}&api-version=6.0-preview.1
you need the artifactId for the request. The artifactId identify a pull request and it could be a part of the response from the Pull Requests - Get Pull Request. But it isn't, the artifactId? is empty.
Therefore, I think, the Author of the article add the following part:
Evaluations are retrieved using an artifact ID which uniquely identifies the pull request. To generate an artifact ID for a pull request, use this template:
vstfs:///CodeReview/CodeReviewId/{projectId}/{pullRequestId}
What should I do with this template? Is there an example? I do not understand this part and don't know what to do.