Original Thread and tip by Ottokerner https://forums.plex.tv/discussion/comment/1291830/#Comment_1291830
If you have installed Plex Media Server on your Windows computer, you add shares on your NAS this way:
Surf to the media on your NAS using the Windows file explorer:
click into the address bar of file explorer once and press CTRL+C to copy the so-called ‘UNC path’ into the Windows clipboard.
Then switch into Plex Web and create a new library.
On the ‘Add Folder…’ tab, click on ‘Browse for Media Folder’
in the resulting window, delete everything from the address bar (topmost)
then press CTRL+V to paste the previously copied ‘UNC path’ to your media into the address bar
click ‘Add Library’