If you save your file within your iCloud folder it should sync automatically for you. It has been a while since I set up my macbook, but my documents folder is within my iCloud folder. So Libre's default saving location is within iCloud for me. I do not recall if this is something I changed, or if it was like that from the start.

But as far as saving anywhere and having iCloud sync for you, I do not think you can do that.