I've played with this and here's my the relevant lines from my working ignition.conf file:
wrapper.share.1.location=\\<REDACTED>\Projects
wrapper.share.1.target=P:
wrapper.share.1.type=DISK
wrapper.ntservice.account=Ignition
wrapper.ntservice.password=<REDACTED>
wrapper.share.1.account=<REDACTED>
wrapper.share.1.password=<REDACTED>
wrapper.share.1.shutdown.unmap=TRUE
wrapper.share.1.startup.max_retries=2
wrapper.share.1.startup.retry_interval=10
For the ntservice account/password I used the local account of the username/password the Ignition service was running under (in my case it was a local account). For the share account, I connected to a domain (computer was not connected to this domain and even on a different network, so it was routed), but was in the form of DOMAIN\username
One note is that your share location cannot end in a backslash - even the Tanuki documentation mentions that this will break it. See: wrapper.share.<n>.* Properties - Java Service Wrapper
I also enabled the wrapper.debug=TRUE for testing and found in the logs the following (wasn't necessary to use debug for this, but when it wasn't working, this gave me the error - just make sure to comment it back out when done testing):
INFO | wrapper | 2024/04/24 14:25:51 | Attempting to map the "\\<REDACTED>\Projects" share to "P:"...
INFO | wrapper | 2024/04/24 14:25:51 | Mapped "P:".
Once working, I just accessed the file such as system.file.fileExists('P:\myfile.txt')
Another note...this does NOT work in Linux. This seems to be Windows onlym and you must be using a separate account for the Ignition service (it won't work running as LOCAL SYSTEM).