F-1 NIW, job position

I can not find my previous poster. Now I know many F-1 students got their NIW approved (Thanks a lot for some kindly answers). I am a F-1 student and want to apply NIW by myself. One question for you sucessful guys. In I 140 form, what job position will you fill in? Postdoc or research associate or some other titles? Do you need a letter from your future boss to support this? Many thanks!