All the major airlines should let you book that ticket, but it will be all on the same airline/codeshare.
I recommending using Google Flights to search itineraries and fares. I recently had an experience with booking a multi-city ticket where Google Flights offered me better itinerary at better prices than what the airline websites offered. When you go to purchase, Google Flights will take you to the airline website.
If you want to search the airline websites directly, you have to do an advanced or multiple search. The exact wording varies from airline to airline and website to app.
For example, if you want to price United, on the app go to Book Flight. At the top of the screen will be four rectangles that say One-Way, Roundtrip, Multiple, and Recent. If you select Multiple you can do the multi-city itinerary.
If you're on United.com, you have to scroll down and select Advanced Search. It's a little hard to see/find on mobile. If you're on a computer, it's at the bottom of the white booking box.