Can I ask the hotel for things like a late check in or a room on a high floor when my customer books through the API?

Yes, you can send special requests with a hotel booking through the API, but treat them as requests and not as something confirmed. The note travels with the booking to the supplier, who passes it to the property, and the property decides whether it can honour it, often only when your customer arrives.

In practice it is usually a free text note you fill in at the time of booking, and with some suppliers a short list of standard preferences such as a twin bed, a non smoking room or a quiet floor. What you can send depends on the supplier behind that hotel. Some accept a reasonably long note, some cut it to a few characters, and a few accept only their own fixed list, so the same request may not be available for every hotel on your portal.

It also helps to know what a special request is not. Anything that changes the room, the number of guests or the price, such as an extra bed, an extra person or a different room type, has to be booked properly as part of the room selection. Put it in the notes instead and the property can refuse it or ask your customer to pay at the desk.

Before you promise anything, check which request fields your supplier supports for the hotels you sell most, and word your booking page so the customer understands the request is subject to availability. For anything your customer cannot travel without, such as an accessible room, have your team confirm it with the property directly once the booking is made.