Hi Adina,
Task Defaults is a referenceable object for the project object.
Within the Task Defaults object, the custom forms associated are listed as a collection (objectCategories).
Since the Original object you are reporting on is the Project, you can "reference" the Task Default object while referencing the custom form collection in two ways:
displayname=Default Categories
listdelimiter=<p>
listmethod=nested(taskDef).nested(objectCategories).lists
textmode=true
type=iterate
valuefield=category:name
valueformat=HTML
or
displayname=Default Categories
listdelimiter=<p>
listmethod=nested(taskDef.objectCategories).lists
textmode=true
type=iterate
valuefield=category:name
valueformat=HTML