There is probably a more efficient way, but this was a quick one-liner that should work.
Get-vm | where { ($_ | Get-TagAssignment -Category 'Showback') } | foreach { $_.name, ($_ | Get-TagAssignment -Category 'Showback').tag.Name, '' }
That will get the VM with the tag from the 'Showback' category, and then list the VM, name, the tag assignment from that category, and then a space, so it's not all cluttered.