To find this you will need your X-Plex-Token instructions here
A list of libraries with the Section Key is available with the X-Plex-Token:
http://{Plex_IP_Address}:32400/library/sections?X-Plex-Token={YourTokenGoesHere}
Gives the following result:
This XML file does not appear to have any style information associated with it. The document tree is shown below.
<MediaContainer size="1" allowSync="0" identifier="com.plexapp.plugins.library" mediaTagPrefix="/system/bundle/media/flags/" mediaTagVersion="1623861150" title1="Plex Library">
<Directory allowSync="1" art="/:/resources/artist-fanart.jpg" composite="/library/sections/1/composite/1624965432" filters="1" refreshing="0" thumb="/:/resources/artist.png" key="1" type="artist" title="Music" agent="tv.plex.agents.music" scanner="Plex Music" language="en" uuid="2318ddce-c8a3-4f52-b5c9-231fd91d2f54" updatedAt="1624965416" createdAt="1624919369" scannedAt="1624965432" content="1" directory="1" contentChangedAt="31077" hidden="0">
<Location id="1" path="/volume1/Plex-Music-m4a"/>
</Directory>
</MediaContainer>
Each library has a key that identifies it within the database. For example, the “Music” library above:
key="1" type="artist" title="Music"
…indicates that the key = 1, the type = artist and the title = Music
the path is also listed as
<Location id="1" path="/volume1/Plex-Music-m4a"/>
Links used (valid as of June 2021)
Plex Support https://support.plex.tv/articles/201638786-plex-media-server-url-commands/