I created the following flow. It almost gets me what I want.
It updates the FMSSID that I want to the new document posted to the Library.
However, the flow is updating the document FMSSID based on the 'ID' created by SharePoint.
My desires is to have my own 'UnqID' used to tie a List Item to a Library Document. I've already created the auto-generating field in PowerApps. The UnqID is uploaded to the List anytime a new document is created. I need that same 'UnqID' referenced in the Patch or Flow so that is also populates along side the new document posted in the Library and acts as its UnqID.