How to configure HTTPS and WSS proxy to an external address in the same port?

[Windol]

I need to configure an instance of https-portal to work as a proxy to a SSL external Node-RED server.
The Node-RED server is listening https and websocket wss in the same port 443.
To work I need to enable WEBSOCKET: true and configure DOMAINS: "node-red.mirror.example.com -> https://node-red.example.com.
I do that, the https://node-red.mirror.example.com works fine, but wss://node-red.mirror.example.com doesn't work.
What's wrong?

[SteveLTN]

This is the code for generating Nginx configuration. Please have a look and see if the generated config meets your needs.