The real question is what advantage you think it will have for the kart engine? The only area I know that really gets deposits would be the piston crown, assuming you’re using real castor oil… Otherwise, modern synthetics and blends burn pretty clean. Auto engines see more mileage at less than optimum mixtures and build more deposits. Seafoam was good for getting the EGR port and other sensors on my Blazer clean, but otherwise, never really thought to use it for a kart engine… And I’d probably just pull the head and use a q-tip with it for cleaning carbon off piston. Or a nylon brush in a drill…or a carbon scraper.
But to answer your question, I’d just remove the airbox and drizzle it into the carb throat while running at a high idle….