I have an invisible disability. I do not really want to disclose what it is, but I have trouble standing up on public transport for longer than a few minutes. I would like to sit down. Now, the bus/tram/train is full. I’m a young adult, barely out of my teens and I look very healthy. I travel alone and have no other visible signs (including a pass or document) or immediately apparent needs related to my disability.
How can I achieve my goal of acquiring a seat without disclosing the nature of my disability and keeping drama to an absolute minimum?
Note that this question is intentionally phrased generally. An answer should preferably be applicable to many different invisible disabilities or illnesses (mental and physical) and should not include suggestions to take a different mode of transport or travel at a different time.
I personally do not take public transport often and am unlikely to have to ask the same person twice, but answers addressing that possibility are still encouraged over ones who don't. (I usually travel by car, but cars can't go everywhere)