If you're logged into the MOB interface (which is really for debugging/troubleshooting/learning purposes), you've already got the address of the vCenter Server 🙂 If you've got DNS, then you can either perform a local forward or reverse DNS lookup
The FQDN of the vCenter Server is stored under VC Advanced Settings and look for the key VirtualCenter.FQDN, but as I said before, you have this info if you're already logged into MOB, so this is a strange request