Residential parking passes don't require ct plates. The guide linked here has instructions for cars not registered in New Haven. https://www.newhavenct.gov/government/departments-divisions/transportation-traffic-parking/parking-enforcement/residential-parking

Resident passes are a sticker. Visitor passes are intended for if you're having visitors for a shorter time, like sometime is coming for the weekend and needs to park. They hang from the rear view mirror.