In order to enable VSS based backups of Hyper-V VMs using Windows Server Backup, you must add the following registry keys to register the Hyper-V VSS Writer with Windows Server Backup. Note that the WindowsServerBackup key is not created when installing the Windows Server Backup feature. This key must be created manually.
Once you have the keys listed above created, you will need to create a String Value with the following.
Name: Application Identifier