Being diagnosed with ADHD 12 years ago and living in Brazil, I have a very different opinion from most answers so far.
I was finishing college when I was first diagnosed, and about a year later I started going to interviews for internship.
Whenever I had a direct interview with owners or direct managers, I made sure to tell them I had ADHD and explain the good and the bad sides of it, also telling them what medication I was taking and what exactly it does or does not. (I wouldn't mention anything when talking with some random HR or recruiter)
Right from my diagnosis I decided I wouldn't pretend to be something else, and anyone interested in maintaining a relationship with me should know about it.
Of course, that doesn't mean I wore a sign saying I have ADHD or mentioned the subject to random people or acquaintances. But whenever any aspect of my condition had a direct effect to people that have to interact with me on a daily basis, I would tell them about it and explain what exactly it means.
I can't imagine having to come up with excuses to coworkers whenever my condition had any effect on my performance. As time goes by, I think all those excuses may start to sound like lazyness or lack of responsibility, and I couldn't bear the idea of people thinking I didn't do something because I didn't care.
Coming clean can also have some other positive aspects, like avoiding anxiety and burnout from not being able to do something you are asked for and keeping it to yourself.
I ask for help whenever I have trouble with work, it doesn't matter if it's related to ADHD or not, but saying exactly what you can or cannot do at the moment can help a lot.
Sometimes this help comes as a 10 minutes or so conversation. More often than not, what is really causing me trouble is the anxiety of not being able to deal with ADHD as I think I should.
Once the anxiety is dealt with, I'm able to handle the ADHD and finish the work by myself.
One advice I give you is to read about what exactly means to have an ADHD brain. I don't consider it to be a disability, but a condition that I have to learn to live with.
There are pros and cons, and you can learn to make the most of it.
And make sure people also know about the pros, for example, if you managed to make something incredibly complex in record time because you hyper focused.
In short, I can't help you with the legal aspects of it and I don't know how people deal with mental conditions where you live, but I can tell you I was always honest about it on my work relationships, even when people had never heard of ADHD before, and it never hurt me in any way. Quite the opposite, I have always found people interested in learning more about this condition and being understanding of my shortcomings.
Never once have I used my condition as a scapegoat, or regretted telling people about having it.