The page especifically states Number of Test. The answer there is a random answer, it does not have to look exactly like that, hence "12345". The instructions, however, say that you must write the Number of the Test you wrote. I don't understand how you can override the specific instructions "write the number of the test you wrote" for an answer that looks more like the candidate number. If they wanted the candidate number they would have asked for it just like they did on the French proficiency part, where they especifically asked for the candidate number. Besides, the test number on IELTS has part of your last name included in it as a reference that it was your test and the especific test you took, general or academic.
