The recommended way would be to do any operations using the UNC network path you used directly and run the Ignition service as a user with permissions to access that network share.
Secondly, it's probably not a great idea to use the root share of a computer even though it's hidden to transfer files. This is a huge security risk!! Create a new share and configure it with appropriate permissions to the location you really want.
Lastly, if you are on a different domain than where the network share is located, you'll have to go about setting it up like I detailed in this post, but as @pturmel will remind you, this share will not reconnect if it's ever disconnected, and if it's not available when Ignition starts, it will delay starting Ignition until the retries and timeouts have been exhausted/expired.