There is a known issue with the Virtual Appliance Agent (VMware, Hyper-V & VirtualBox) where the serial number is not displayed at the end of the boot process. This will be fixed within the next few weeks.
In the meantime, just use the chat icon at the bottom right of the Website or App and give us the public IP, private IP and MAC address of the VM. With that information, our support team will be able to find the agent and confirm its serial number.