I also have ADHD :
1) Anki is a life saver for interviews: in the short term, it helps me memorize and review stuff about the company in advance. In the long term, it helps me memorize IT related stuff, and even leet code exercise. This maximize the chances I'll be able to answer without having to search through my memory.
2) I try to engage with the information I need to memorize in different formats (Text, Video, Image, Mindmaps, etc...). This helps me build multiple paths in my head to find the same information, which then makes it easier to remember it on the spot. Betterexplained has a pretty good framework for that, ADEPT : Analogies, Diagrams, Examples, Plain English, traditional definition.
3) Anxiety makes the symptoms worse, so I try to jog and meditate on a daily basis. Journaling is also extremely helpful, since it allows to get things off your chest and mind.
4) The brain fog is more intense after doing something stimulating, so if I know I need to focus, I try no to play video games, watch movies, etc...
5) The state of what's in my head is always kind of like the state of my apartment, so I try to keep it clean
6) A bad diet makes the symptoms worse, so if yours isn't good try to follow the usual recommendations.
7) Not eating enough, and not eating often enough can have a huge impact. This can especially be a problem for us since the medication can stop you from feeling hunger.
8) Similarly, when you're not hydrated enough, your ability to focus drops, and the first signs of a lack of hydration isn't thirst, so make sure to regularly drink water regardless of whether or not you feel thirsty.
9) Practice out loud different ways of responding to questions commonly asked, and questions you feel they might ask you. That's kind of like what I do to retain information: Building different paths to the same information so that I won't freeze during the interview.
10) You already know that you'll never be able to be perfectly prepared, so prepare in advance things to say when you're asked something you're not able to answer on the spot, things to gain time as well as things to say when you can't answer.
11) Since you still have some leeway, treat the first few interviews you'll do as experiments, and after each one of those, look at the results, identify what went wrong, make some hypothesis on what could you do to fix it, and try it out in the next one.
12) Join a community of people sharing your struggles.
13) Get your 8h of sleep