Dear Amy,
I am Renzo, as an Independent Advisor and Office user, I'll help you with your question.
At this point is not possible to select specific folders from any location to sync it with OneDrive, basically, if you want to backup a specific folder you need to move it (or copy) inside OneDrive main folder, default path will be:
C:\Users\YourUserName\OneDrive\
But I suggest you to send your feedback to PG (product group) through the Microsoft OneDrive UserVoice, is the way to communicate with them, they always see ideas and suggestions from community users in order to improve the app.
https://onedrive.uservoice.com/forums/913522-on...
Any doubt, let me know and have a nice day.