I've never put it on an application (or told any company I worked for). And I wouldn't for precisely the reason you said - discrimination. It's very illegal for them to deny someone because of a disability, but the disability will never be the reason that's given. They don't state specific reasons in the notice, they say they "are pursuing other candidates", etc. And if you call them on it or sue them, they can pull out a stack of resumes that are similar to yours. "We didn't hire any of these candidates. Their resumes are similar to yours, none of them have ADHD, and they didn't get hired either. The candidate we chose had a resume with xyz, our decision had nothing to do with your disability, it was based on xyz."

Once you're hired, it's still a slippery slope. If you tell the company, you do get some protections. They can't fire you unless they provided "reasonable" accommodations for your disability. Depending on the company, they might be great about it and provide some real meaningful structure and support. On the other hand, some companies might only see the the potential cost and stereotypical ADHD issues. They might only provide token accommodation and call it "reasonable". All the while just waiting for any valid reason, no matter how trivial, to fire you.

Most of what I've read has suggested serious consideration before you notify your employer. First is really knowing your individual ADHD symptoms. (The employee who occasionally gets sidetracked when speaking, or who's always just a minute late can be forgiven. The employee who impulsively tells the boss to shove it won't be). Then look at things like what accommodations they've made for previous employees, possible suits brought against them, etc...