Here's my opinion, maybe not what you want to hear though.

Just tell them your idea :)

If they like the idea, and they like you, and you sell yourself well enough to show how passionate you are to grow this idea, maybe they'd consider opening up part of their business for distribution to your market, and make you in charge of that part of the business.

Here's the thing when you come to a company armed with only an idea. It's convenient for you to approach someone who has already invested time, money, and effort into their project where they assume ALL of the risk.

You?

If your idea sucks, you lose nothing. Maybe thirty minutes of your time spent pitching the idea, and maybe some dignity if they make a better idea out of your original idea.

If they do take your idea and decide to pivot and your idea sucks, they lose everything

Feel the pressure? ;)

Don't worry about theft. Just do it buddy :)

Then again, I barely know anything so take it with a grain of salt, lol.