can appearing studends fill the neet exam form

Yes. And their code will be 01. Code 01 says: A candidate who is appearing in the qualifying examination, i.e., 12th Standard in 2020, whose result is awaited, may apply and appear in the test but he/she shall not be eligible for admission to the Undergraduate Medical Courses, if he/she does not pass the qualifying examination with the required pass percentage at the time of first round of Counselling.

But such Candidates admission will be provisional subjective to the outcome of their 12th final results. They have to secure the aggregate required for their Category in Physics, Chemistry and Biology. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST.