You cannot specifically apply for the next written exam. All you have to do is wait for the invitation from Mahantesh (and that turns out to be a random process). Till then, you can send him multiple emails with your CV requesting for an opportunity to sit for the exam.
WARNING : I don't think SpiceJet has a fair selection process. But I'm guessing if you do extremely well in the test, you may be called for the interview.
Good luck.