Booking for the same evening
A same-day booking is arranged exactly like any other, with the deposit rules decided by who is asking and by where the room is rather than by the hour.
Nothing about booking for tonight is a special case. The same four facts are settled in the same thread, the arrival window is the same forty to sixty minutes for a central address, and the figures are the figures printed in the profiles.
What decides whether something goes ahead
Two questions, neither of them about the clock. Have you booked here before, and is the room inside the city. A returning guest asking for one or two hours at a city address sends nothing in advance and settles the whole figure when she arrives. Everyone else sends half, including anybody booking here for the first time.
The extra line in the thread
Because a same-day session is performed within the period a distance agreement would otherwise allow for withdrawal, the house asks you to write one sentence in the thread confirming that you want it carried out straight away. It takes a moment and it keeps the arrangement tidy. The deposit and cancellation page explains where that sits in Dutch law.
Getting a fast answer
Write once, write everything, and do not split the question. The address with its floor and room number, the hour, the length, the figure, and a name if you have one. If you have no name, say that the hour matters more than the name and ask who is free, and the reply will tell you. A phone left face up after that is the last thing required of you.