[Zope] Simple q'n but driving me crazy

I tried several things but none gave a good result :-( I've a property sheet call InstanceConstants. Assume there is a property called 'Category'. Now how can I automatically fill this value with a string, thus not through addForm (user intervention)? eg. Category = 'TEST' I assume this piece of code has to be placed in the _add method, but how does it look like? <dtml-call "REQUEST.set('Category','TEST')"> doesn't work and all my other trials with using the propertysheets names etc don't work either :-( Tom.