Yes, you can add a special request to most hotel bookings made through our hotel booking API, but it is a request and not a guarantee.
When you book, there is a field where you or your staff can type a note such as a quiet room, a room on a higher floor, twin beds instead of a double, or a late arrival time. That note is passed to the supplier and then on to the hotel, and the hotel decides on the day of arrival whether it can be done, based on what is free when your customer checks in. Some suppliers accept free text while others only accept a set list of standard requests, so what you can send depends on which supplier the room comes from.
A few things are worth telling your customer in advance. Requests for an early check in, a late check out, a baby cot or an extra bed are often treated as paid extras by the hotel and settled directly at the property, so they cannot be confirmed at the time of booking. Free text notes are also sometimes not read if the hotel staff do not read the language you wrote in, so keep the wording short and simple. Anything that truly must be guaranteed, such as a ground floor room for a guest who cannot manage stairs, is safer handled by contacting the hotel yourself once you have the confirmation.
Before you promise anything to a customer, check which request options appear on the booking screen for the hotel you are selling, and note on your voucher that requests are subject to availability at the hotel.