FREETEXT searches are all about meaning, so SQL Server looks in the thesaurus to find matching words. If you edit the thesaurus so that 'Projectnummer' is a synonym for 'Project', it will find it. If the term isn't in the thesaurus, it's not going to find it.

But if you want to use prefix terms in searches then limiting the search terms and using this form will work:

CONTAINS (Textfield, '"Project*"' )

I'm not an expert but that's my understanding.