Hide private IP address?

[upd108]

Hi
I setup nginx proxy manager successfully and I can access my application via public address www.xyz.....com
The application runs on a private LAN 192.168.xx behind nginxproxy. Everything OK, except the following.

If a user accesses the application via www.xyz.....com the application sends an Email alert to the user that says:
You accessed your account via IP-Addr: 192.168.x.143
But the alert should report the public WAN IP only not the user client private IP of the user PC.
To my understanding the application should not even see the users private IP, but only the WAN public IP of the user?
Is this an issue that can be changed on the nginx-proxy, or is this something that I have to configure on the application behind nginx-proxy?
Thanks for any help!