All you have to do is before the actual test,just let them know,and also bring with you proof of having to be on this particular med.as long as any med has been legally Rxed by your doc,there should not be any sort of a problem.and they do not have the right to force you to divulge personal medical information.it will just be noted for the test that you are taking a legally Rxed med that was found in your urine,thats all.
All the proof they need is the actual Rx bottle.that should take care of it.they,in most cases,will just have you write down any actual meds that you are on before the test is done so when it is actually found in your urine,it will be noted that it is legally supposed to be there because you have proof of the Rx.
in most cases,unless there is some very specific need because of certain job requirements(very very rarely)no employer can even ask about any actual medical conditions you may have and cannot ever(force you) obtain your medical records,from you or anyone else.they are off limits to your employer.
just bring that bottle with you the day of the test and state beforehand that you are taking it and you should do just fine.good luck on that new job,Marcia