Supposing to write anything on a banknote, most likely millions of people will touch it and maybe half of those will read what is written on. Imagine to write a web URL on ten-dollars bill, above all the people who will read these two lines, how many will reach your URL address?
This is what made a friend of a friend of mine. But, is it illegal to write on money? Yes and no, it is not persecuted in many countries if that writing does not alter the banknote itself to confuse its true value.