How to access demandware support portal?

I have bought new B2C realm for one our customer. I got the urls for instances development, staging and production. Through Account Manager credentials, I am able to login to all the 3 instances. 

I am facing some issues with creating sandbox and tried to login into demandware self service support portal - https://support-demandware.force.com/customer/login

Its not accepting my account manager credentials and I am stuck with moving forward. Can any of the Admin know about this issue, Please help me to resolve it.