Can you go in the Army with ADHD

From what I know it depends on the severity of it, however this is up for the doctors to decide when you go for your medical screening during the application process.

Hi, I've noticed that its part of the declaration though, so if I declare it, will that immediately disqualify my application or will it be able to progress as far as a medical?

If you put it on the declaration it means that they will look into it further before accepting your application, it wouldn't automatically disqualify you it just means that they will have a closer look at it. However if you don't declare it, it will get picked up on the medical which may delay the process further.