How to host webapps alongside the server.app based web tools?
Server.app provides a Websites service which can be used to publish your own web content. This service provides support for web apps (alongside normal websites) through the use of webapp.plist(8) and webappctl.


  You can define the web apps managed by webappctl by creating plist files and placing them in /Library/Server/Web/Config/apache2/webapps/. For information about these plist files, enter man webapp.plist in Terminal.


https://help.apple.com/serverapp/mac/5.2/#/apd85e35fbb-8fc2-44ed-855f-bf4e67374de6