The above method is documented as working as well as the use of a configuration file (the method I use) for the current stable release of the extension (see here). Not sure about the alpha release though.
Path to config file on QNAP: /Container/container-station-data/lib/docker/volumes/[Optional App Name]_roon-extensions/_data/node_modules/roon-web-controller/config
Shouldn’t the question better be posted in the respective Roon Web Controller thread (stable|alpha)?
I see it’s already there: