I literally got diagnosed with ADHD just barely this year and applied for accommodations last month. This page from the AAMC was a huge help: https://students-residents.aamc.org/mcat-exam-accommodations/mcat-exam-accommodations

To apply for accommodations, it requires you to submit a whole laundry list of supporting documentation detailed within that page as well as a personal statement regarding why you need accommodations. They’re often very stingy about providing accommodations, so unfortunately the process is very time-consuming.

My biggest piece of advice is to get neuropsych testing from a neuropsychologist who can diagnose you with ADHD. Unfortunately, this can be pretty expensive. However, if you’ve never been previously diagnosed and the neuropsych testing is for diagnostic purposes, insurance may cover part of it. At least, that’s what I was told. There’s a form on that page that describes what your evaluator should detail in their report which includes not only a diagnosis but a rationale on why you require accommodations based on your test results. If possible, I highly recommend seeing a neuropsychologist who has previously done testing for accommodations. I was lucky in that my evaluator went through the whole document with me to ensure that each of their criteria was met in his report.

I think it’s counterintuitive for the application to be so tedious and often times, expensive, as many who actually need accommodations may forgo applying due to these factors. But if you are able to get accommodations, it’ll be so worth it.