Firstly, you don't have to put in a number. I didn't for either of my tests. Secondly, I'm pretty sure the ADI number is your instructor's personal reference number.
Pretty sure it is also used to stop the same ADI being booked in for multiple tests at the same time. My instructor said something to that effect anyway.
As others have said, I don't think it is essential [esp as you can take a test in your own car/with an ADI], but your instructor should be able to tell you what it is.
You don't have to enter it. I queried it with my ADI once and she said that if she takes someone for a short notice test because they didn't have an instructor, she wouldn't even leave her ADI badge in the car.