Question Sails.js Socket.io Config

[EarMaster]

I'm trying to run a Sails.js application on my Plesk Obsidian server (running on Debian 10). The Sails.js app itself is running and available, but I struggle configuring the Socket.io route. I disabled proxy mode and tried to add the Upgrade headers as described here: Using NGINX as a WebSocket Proxy

I added the following location to the Additional nginx directives section of the domain:

passenger_sticky_sessions on;
location ^~ /socket.io {
proxy_pass http://127.0.0.1:9856;
proxy_http_version 1.1;
proxy_redirect off;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Frame-Options SAMEORIGIN;
}

Unfortunately this doesn't work. I suspect the problem may be that I cannot connect directly to the port of the application, but I need to connect to the port passenger is assigning the application. But I don't know how to find out this port.