When you buy your CentOS 8 server this will be automatically provisioned and built for you. From there you should be able to find it in the Manage VPS section of your account. Selecting Options > Manage from the Manage VPS page will take you to the overview for your VPS. From here you can find the root login details at the time the VPS was provisioned. Please note that if you update these details, the UI will not also update - so take care when changing the root password. This article goes through what you should do to create a solid base for your new server, before installing or configuring any software or services.