Multi-city or open-jaw search functionality is not currently available in the app, but it's a feature that we hope to offer in the near future! You can always search and book multiple one-way flights to make up a multi-city trip, but please note that they would be separate transactions. You can search for one-way flights by selecting "One way" at the top of the search page:
Does Hopper Offer Multi-City or Open-Jaw Trips?
Written by Hopper Support