When you make your room arrangements, tell the Disney arranger that you will be celebrating an 18th Birthday. If you have no reason to reach out to the hotel before you arrive, simply tell the check in staff that you have a birthday girl. They will supply the Happy Birthday pin. I celebrated my 60th Birthday down there two years ago and it was the greatest experience. Besides the hundreds of birthday greetings, you get special desserts at some of the restaurants and there may be a special treat every once in a while. I got a sword, pirates eye patch, snow globe from bus drivers and shop workers and autographed birthday cards from the characters at the Crystal Palace and Chef Mickeys.. You never know what may happen. I wore my pin for the entire week I was down there. Last year my wife and I went down for our 41st Wedding Anniversary and got pins to wear the entire week as well. Just ask at the font desk and they can help you. You can also arrange for Mickey to call you in the morning to wish you a happy Birthday on your special day.