I got a late diagnosis too but it was more of my psych's idea. I am also autistic so I figured all of the executive functioning issues were due to that. I explained to her how bad it was, how I could never start or finish things even if I wanted to, and she started to suspect ADHD. She gave me a short screening test and then kinda waited for months to get more info from me and confirm. The medication is helpful, but things that frequently occur in people with ADHD like depression and anxiety can also contribute to lack of focus and so it is possible to still have trouble focusing even after treating the ADHD symptoms.

I agree with DrComrade's strategy of just honestly describing your symptoms. If you have ADHD then that would probably be convincing. Some aspects of it might be hard to notice since you would've had it since you were a kid so the people suggesting to look at the criteria and some online resources have good ideas too.