HK visit Visa for Nepalese citizen

Hi, I'm from Nepal and want to surprise my boyfriend on Valentine's day, who is a resident of HK and is living there. What I'm confused about is the flight details and itinerary the application form requires, like obviously I can't book a flight without getting the visa first, in case I'm rejected, right? So, how am I to complete the form without first booking the flight or is it compulsory that I first buy the plane ticket and then apply for the visa? I'm confused. Please Help.