Siena: That looks more like a transmission breather valve, not a filler port.

One of the reasons you need to read your owner's manual from beginning to end. Most errors with regards to automobile use and maintenance stem from ignorance. Like some folk running 60 psi in a 195/50x15 tyre, because they fail to read their Owner's Manuals.

Farayolas: thanks sienna and zim drill
but i am still confused whether it is a transmission breather pipe or topping up filler port

Don't be confused. I don't recommend filling the gearbox via the breather port. The filler port is also where the level is checked. The vehicle has to be raised, and both sides level. Once the level is correct, the oil will begin to flow from the filler port, and that's when you put the plug in.

Filling via the breather is bad practice. You have no way of knowing if you're over-filling the gearbox. Filling via the breather, speedo drive recess (after removing the speedo drive) or via the reverse light switch aperture (after removing the reverse light switch) only makes sense, if the gearbox is completely empty. So let's say it takes 2,2 litres, you know you're putting 2,2 litres in.

For topping up, you really need to fill it via the filler port. If you over fill it via a breather port, the gearbox will hydro-lock whilst in service. This is whereby excess pressure builds up within the differential housing. If it's over-filled, the breather will not vent the excess pressure off quick enough, and if it hydro-locks, the differential case can explode.

If you tell me exactly what year your Honda Prelude is (not 1990 - 1991) and exactly what the engine size is (not just saying "4 plug" I can tell you exactly where the filler port is.