Nginx Proxy Manager - Keep Alive Setting

I have a question about which I am racking my brains. The following scenario. There is an application which displays a white page via the browser internally via a port and a path /keep-alive. Externally via the configured Nginx Proxy Manager the application is reachable via FQDN.

So a proxy host has been set and a stream for the port. If I now access the port and /keep-alive from external I get

CANNOT GET keep-alive

I wonder, is this a setting that needs to be set on the Nginx Proxy Manager (and if so, what the parameter for it needs to be) or does it need to be set on the web server by the software developer of the application? The web server that runs the software in the background is OpenResty (based on Nginx Core).

Thanks for your time!