Github API: How to get issues closed by a PullRequest

I am looking to write a script that sends notifications about what issues have been resolved when a PR is merged.

I am able to get the PR object, but I only see a reference to an issue (using the closes #\<number\> notation) in the PR description.

Is there a different way to determine this relationship? 

No, there isn’t a different way to determine the relationship. You can see the more precise criteria in our help documentation.

I hope that helps!