Passing blueprint properties to externalWFStub.postprovisioning

I am using vRealize Orchestrator with vRealize Automation. I have added the externalWFstub.postprovisioning to a blueprint and am calling a powershell script on my powershell host. The script gets executed and errors because it is not finding the parameter passed to the script.

My script command line is:

scriptname.ps1 -computer computername

Computername needs to be the NetBIOS computername the VM was provisioned with. The computername is derived from a Machine Prefix.

How do I pass the blueprint MachinePrefix derived name to the blueprint property. I have tried

scriptname.ps1 -computer {VC:MachineName}

scriptname.ps1 -computer {Computername}

scriptname.ps1 -computer {Hostname}

Thanks in advance for an assistance