I didn't say it doesn't work, just that the question has this flow aspect that was not replied to. Let's read the part of the question by @bassman99 again carefully, it is about Power Automate:
"I've seen an example of how to do it in the same flow as when a folder is created, but I can't figure-out how to do it for an existing folder."
I had exactly the same situation. I knew Create folder action is returning the folder ID, but how about when the folder already exists and you need that ID, for creating a sharing link for example?
Get folder metadata using path -> ItemId

The Folder Path needs to begin with "Shared Documents/" (if the language is English).
I still have to dig deeper into SharePoint actions, why some require ID and some path. Also, actions that ask for document library, may have different folder path to fill in that with those that only ask site address. I'm trying to find the consistency, or proof of the lack of consistency 🙂