If you are OK with removing it, I use a variety of laundry products including powdered detergent and "Biz" or "Oxi-Clean" which have cleaned many hundreds of keys and only failed to clean 2 or 3 after an hour soak in a strong solution.
If not removing the key, I use denatured alcohol on a Q-tip, and if that doesn't get it, try a white vinyl eraser (eg "Magic Rub") but gently.
Lastly, carefully, if it is some sort of adhesive-related product, using a citrus oil organic solvent or naptha (old-fashioned "lighter fluid") might work, but these last 2 are harsh, and I can't guarantee that they won't hurt the plastic.
Try each one gently, slowly, and remove all the residue after each attempt.