I agree with Kai and W. Are's answers and while their answers outline why you should not get your hopes up, I'd like to highlight a few examples of animes that show that this isn't necessarily impossible.
First up are Heroman and The Reflection. Looking at the writers, you'll notice that they were both written by Stan Lee, creator of many well known Marvel comics.
As a second method, I'll bring up Neo Yokio, an anime that was harshly criticized but shows another possibility: Netflix. Netflix has been investing a lot of money into not just getting animes on its platform, but producing them as well. This one was created by the lead singer of a band, but I'd like to think that Netflix could open up the possibility of Western people getting more involved with Japanese studios.
Finally, you can make your manga and hope it takes off. As Kai mentioned, they have hundreds, probably thousands of ideas they can consider. Outside of being in the industry (and having shown success), the only real way to get your ideas turned into an anime is by your idea being a success beforehand.
Some examples in this last category include Radiant, Howl's Moving Castle, and Tales from Earthsea. Do note, however, that in the case of the Earthsea, it was changed enough, for Earthsea's author to not really consider it her story anymore.
This brings us to Radiant. This is a comic that takes a lot of inspiration from manga and thanks to the artist of One Punch Man, it got picked to become a part of the Euromanga Collection. A safe assumption may be that this became an anime thanks to having presumed success in Japan.
I think what this all shows is that it's not about having a good idea, but like W. Are said, giving the studio a reason for them to make it in the first place. And as my examples show, this reason could be you being a person of large influence, getting the right connections or your idea having success in its medium (whether a comic, novel, or perhaps some game).
I wouldn't get your hopes up with just the manuscript/idea, but do know that it's not outside the realm of possibilities for your work to get an anime adaptation.