First, unless your condition or the medicine impacts your ability to fully participate, or poses an issue to your continuing health or safety, I would be reluctant to volunteer the information. You are better off asking your doctor whether you should report this , rather than this Forum.
If asked, you should always tell the truth. Perhaps someone that has attended NASS recently can tell if there is an initial medical screen when you first check in . (I would expect there is, and preliminary screens usually ask what medications you are on).
Finally, there are numerous threads on this Forum discussing skin conditions ranging from Eczema to Acne (and probably others). I am not going to venture to provide medical advice (particularly when OP doesn't identify the condition involved), but will suggest that you be proactive ...starting with reviewing the applicable Medical Regulations with your doctor to determine whether the condition is disqualifying. If the condition is disqualifying, you may be able to work with your doctor to develop a course of treatment that gives you the best shot at a waiver.