Hello,
where are .you exactly being asked for a username and a password?
hat
if it's on the direct console screen, I mean if you are connecting directly to the server via a KVM or by plugging a keyboard and a screen, and you see that orange/black screen, you should enter "root" as the username, and the password that you've specified in the installation process of ESXi.
if you are trying to connect to the server via the vSphere client (the desktop client program), you should also enter the same credentials as in step one.