Is subdomain forwarding with mask can work even I am using custom name server? According to this https://www.godaddy.com/help/manually-forward-or-mask-your-domain-422
"To forward or mask your domain name, you must use our nameservers. For more information, see Change nameservers for my domains."
You wouldn't be able to use our forwarding services unless you were using our nameservers. If you want to use custom nameservers, you'll need to check the DNS management at those servers to see if there is a forwarding option available.