Hi Bo,
REST API in general (not Clarizen's particularly) requires usage of HTTP methods other than GET.
CreateFromTemplate expects POST method (says the documentation).GET may work here, but I can't guarantee it.
Anyway, the entity parameter has to stay as it is, i.e. entity={"Id": "/Project","Name": "9998 Program"} as JSON representation of entity object. You can't just use the properties.
You may also need to URL encode that input.
Hope this helps,
Ophir