If Document Root is changed at Plesk > Domains > Example.com > Hosting Settings it will automatically change the Document Root in node.js configuration at Plesk > Domains > Example.com > Node.js
Is it possible to keep a static site and node.js application within one domain?
To keep a static site and node.js application in one domain set Document Root for the static site as a subdirectory of the node.js application root directory in Plesk > Domains > Example.com > Node.js as an example:
Document Root: /application/httpdocs
Application Root: /application
and configure the same Document Root in the Plesk > Domains > Example.com > Hosting Settings.
To get more information refer to the official Plesk documentation Node.js Support