Do you already have an ADHD diagnosis, but no treatment yet?
Has treatment been declined due to whatever you need rehab for? (I've been looking into ADHD a lot as I strongly suspect that I have it too, and it seems that accessing ADHD treatment while still using anything else will be a big no no for safety reasons - potentially dangerous interactions plus unfortunately you'd been seen as high risk for abusing ADHD medications)
A number of ADHD people I've asked about this on other message boards, all said that they reported their drug or alcohol use as being in the past in order to access ADHD medications.
I've never been in a rehab, but have used 'services' and I would guess that TCITH's suggestion is correct.
If / when you get your using under control then hopefully you'll be able to look at accessing proper ADHD treatments further down the line.
Untreated ADHD is a common risk factor for substance use, as I'm sure you know. But the danger is that substance use can prevent access to those meds that would help stop any problem using.