There is no secondary prompt for your personal statement on AADSAS. Here is what the instructions say for the current cycle though:
The personal essay provides an opportunity for you to explain why you desire to pursue dental education.
It is recommended to compose your essay in a text-only word processor (e.g., Notepad), review your essay for errors, and cut and paste the final version into the text box. The essay is limited to approximately one page (4,500 characters, including spaces).
What should you write?
The Admissions Committee members who read your essay are looking for individuals who are motivated, academically prepared, articulate, socially conscious, and knowledgeable about the profession. Write about your experiences and any qualities that will make you stand out. Check the ADEA Official Guide to Dental Schools for ideas about essays.
Hope that helps. Good luck.