I work in advertising.
Most companies with a significant ad budget will go through an ad agency, others will do everything in-house. Regardless, there will always be something called a "brief" (or an equivalent) that outlines the specific business problem that needs to be solved, and the general tone that the creative must achieve to work alongside how product is currently positioned. These are (ideally) very detailed, multi-page documents that cover strategies and target demos and consumer buying habits and other information that you are simply not privy to.
It will be a one in a million shot that your ad will match their goals, regardless of how clever it is. And yes, "clever" is all too often a replacement for "effective."
You "great idea" is only great by your standards. Not the company's, not the brief's.
Plus, if a company is using an agency, they aren't going to use you. Full stop.
That isn't to say you shouldn't storyboard it or even shoot it.
If you want a future in advertising, this is what's called "spec work" and most people get their first creative jobs by having a portfolio ("book") full of specs for companies they don't actually work for. If you can execute this and a several more ideas, and you can justify to a advertising hiring manager how it solves what you imagine to be the company's business problem, then you should absolutely go for it.
But don't even think about contacting the company about this. The only companies that wouldn't roll on the floor laughing at you are the ones too small to afford a tv buy.
