My Book Word Edition + Twonky Server: How to access the root folder of the NAS

I own a Western Digital 'My Book World Edition' (that's a simple NAS) for some years now, mostly as backup and central data access point.
After having purchased a smart TV, finally I was motivated to activate the TwonkyMedia server to stream media via DNLA.
Everthing went fine, however only the media files in the \public folder of the NAS were indexed by Twonky and I did not manage to get access to the whole NAS via the Twonky browser. Pretty bad as I was used to my private well-known folder structure and not willing to change it.
I did not manage to fix the problem for some time, and for some reason I did not find anything useful on the net, until I came across this topic on WD community forum:

http://community.wd.com/t5/Other-Network-Drives/USB-Media-Share-TwonkyMedia-Server/td-p/28117

Here the main information with the single command to adjust the start location, thanks to user garyg:

This is because the shares start at /DataVolume/Public.  If you change the default start location to /shares, or maybe it's /Shares, you will be able to access anything which has been defined as a share on the MyBook.

The command to fix the USB sharing problem is:
         http://<nas>:9000/rpc/set_option?contentbase=/shares

where <nas> is the name or IP address of the My Book.  This command is entered into your web browser's address bar.  The browser will respond with a page that says "/shares".