NVM. I found my answer on VMTN. Here is what I did... I ran the following command to add vCenterServer property to Virula property .
New-VIProperty -Name vCenterServer -ObjectType VirtualMachine -Value {$Args[0].Uid.Split(":")[0].Split("@")[1]}
and then
Get-VM -Name vmname | New-TagAssignment -Tag "prod" -Server vCenterServer