Are You trying to create a ticket via web portal? I can’t modify creator data during ticket creation. Maybe You will do this by scrip? If You create ticket by rt portal, You are rt user and it might be first of condition for Your scrip.
Wouldn’t you need to either set Owner or Requestor instead of Creator (which doesn’t really exist as a field for as far as I know, unless if you’ve added a custom field to support the extra information).