How do I license a product to a manufacturer through their advertising agency?

I've never been a "middle-man," and I'm confused because ad agencies charge by the hour, and I don't want to split their hourly profits, but rather the manufacturer's sales revenue. I want to approach the agency with a product idea that leans towards packaging/advertising, with the agreement that if they pitch my idea to the producer, the producer can only take the idea if they agree to split revenues with me. But that's impossible since it would mean the agency being able to do the same, but of course they only work by the hour. So is my only option to contact the producer's advertising account executive, and pitch my idea to that person directly instead of their agency, unless I want to limit myself to splitting the agency's hourly income?