sorry, do you mean there is something like a checkbox for "[] hotel []airbnb on the entry form itself?
on our last year trip to Osaka, we were asked to write down the complete address and contact nos of the hotel we are staying.
this year, i opt to try airbnb and am aware of the airbnb situation so i asked the host, and his answer was reaassuring, i even asked if its ok to show the map and address of the house, he said yes no problem, people from all around the world do it and they had not any problems.
I did googled it as well (airbnb immigration) and people seemed to have no problem with the immigration. their primary job is just to ensure you have an address where they can reach (you just in case).
The only other thing you probably do NOT want to do is to put a fictitious address or lie in the face of the officer, because if you do, you might get into the point where the officer would ask just a simple question and you begin to stutter (because you were lying), and office begins to doubt you and asks more question and you answer twisted things or make things up.
