they need to know for health and safety reasons, but likely it won't affect whether or not you get 'diagnosed', if anything it will affect the approach the doc takes towards treating the issue at hand, in most cases, if there's prevalent drug abuse and the doctor believes ADHD is the issue at hand, they'll treat the drug abuse first before considering medication, but it still doesn't rule out medication all together. In other cases, they'll opt for non-stimulant medication to begin with; but proper medication reduces the risk of drug abuse for most ADHD patients, there are some complications though depending on the person
honesty is the best policy, I had an issue with abusing ritalin IR after a couple months with it, a few things led to it, I have a history with drug abuse (which I also let my psychiatrist know fully about) so I fell into old habits in a way, but the way it started was that I was hitting rebound/crash states sooner and as such started redosing sooner, that quickly got out of control
I told my psychiatrist about this outright, because truth be told although I was the one to blame for the misuse/abuse, I didn't WANT to be abusing my meds, There was nothing positive about doing so and it was a major slip up on my own behalf as a result of trying to maintain the functional state with better coverage; which quickly got out of control.
so we discussed ways to deal with it, now i'm on Concerta and don't have the same issues, take one a day and have no second thoughts about it.
My psychiatrist specializes in drug abuse and adhd so, I figured it'd be best to just tell him these things with full honesty because I feel myself to be in safer hands when it comes to his decision making, he's one of the top rated in my city and yadda yadda, but yeah he basically told me that if there's issues with drug abuse especially habitual/addiction levels, then patients with ADHD need to spend a good half a year or so away from that, this was something I did myself prior to going in for diagnosis anyway, stayed sober for a good 6-7 months with very rare, occasional use, my primary culprit was self-medicated cannabis which I was chronically redosing