In the the address field write "onward connection to Canada on AC124 and show them your AC BP or AC ticket (if you don't have an AC BP).

Also, you can use the USA Passport holders line rather than the Foreigners line. This will save a lot of time.

Have fun and if you have more questions regarding the connection process the state you connection city, likely a lot of FTers will know a few ins and out wrt the particular city and terminal you transit.