Hello @Connor A
I know you can create a new calculated column that combines multiple columns to create the unique key using Power BI data transform. However, I have reached out to the product team for any suggestions/insights. Once I get any insights, I'll update my answer.
EDIT 18 October 2023 See Understand usage details fields - Microsoft Cost Management | Microsoft Learn, you'll notice that there are some columns flagged with a 1. You can use these fields to build a composite key.
¹ Fields used to build a unique ID for a single cost record. Every record in your cost details file should be considered unique.