PWDM - Transmittal package ID to use attribute

Hi

I want the transmittal reference to include a package reference instead of the cloud project number - I thought I could replace the prefix ProjectID as shown below

Package type Prefix Preview
Transmittal {ProjectID}-TR-

With {$DocumentAttribute.GetValue(AttributeName)$} but when I review the transmittal prior to submission it just shows the reference as {$DocumentAttribute.GetValue(AttributeName)$}- TR-001

can it be done?

john