Any visitor who is not staying the night must pay a 5 euro ($5.35) entry fee online before walking into the lagoon city on April 25 - a national holiday and the first of 29 days this year
Wearing a navy suit on Wednesday, the prince answered the phones alongside traders in fancy dress costumes for the charity day where all ICAP's profits from 60 offices are donated to more than 200 charities worldwide.
ICAP, buyers and sellers of bonds, currencies and swaps, began their charity day in 1993 and have raised over 120 million pounds with celebrities brought in to help raise money.