Proxy authentication

I have a self-hosted VPS with proxy service running, however it is behind authentication. I can use the IP address and port "IP:Port" in the custom proxy settings to browse but, I need to fill in the proxy credentials into the login pop-up every time.

Is it possible to authenticate automatically with a specially formatted url that I may have missed. I tried "user:pass@ip:port" but it's considered invalid.

This would be very helpful.