Forwarding with Masking Manger Server is not accepting user name n Password

The problem we are facing right now we recently have purchased a domain from Godaddy for the Manager server edition we are going to redirect the webpage to our server with static IP when are forwarding with masking the manager login page is opening but it’s not taking the credential and when are doing only forwarding it’s accepting the credential but showing the static IP address I had a word with Godaddy technical support person they are saying to contact the software company please suggest .

screenshot for Forwarding with masking

Why are you forwarding to an IP instead of setting a DNS type A record?

I don’t know what “forward with masking” even means… is it reverse proxy? iframe method?

The standard way is as @Davide suggests set up your A record for subdomain so it points to IP address of your server.