Hey,
yes, when using proxies you have two ways of authentication, to kinda tell the proxy "hello, it's me"
You either use your VPS IP in the Panel to auth your IP or just use the username, password method if your proxy supplier provides you with that information.
If you don't auth your IP or don't auth with username, password you won't be able to connect to your proxy.
I prefer using username&password because you just set it once and forget about it when you either change VPS frequently or have a dynamic IP at home, but if suppliers don't provide you with username&pass, just search for the section where you can auth your IP to give yourself (or in this case MP) access to the proxy.
Regards,