I've solved this!
I can't find the stackoverflow question that helped me, but they gave me the idea.
git log --format=format:%s -1
Specifically, I have a bash file that does the following as part of my jenkins pipeline.
echo "COMMIT MESSAGE" `git log --format=format:%s -1`;
In my PR build, it prints the following:
2021-10-07 12:56:43 COMMIT ...