Can my customer ask for a late arrival or a particular kind of bed when they book a hotel on my portal?

Yes, in most cases your customer can add a special request when they book a hotel room, but a request is never the same as a guarantee.

At the booking step there is usually a free text box, or a short set of tick boxes, where the guest can ask for things like a late arrival, twin beds instead of a double, a quiet room, a high floor, or a cot for a small child. That note travels with the booking through the API to the supplier, and the supplier passes it to the hotel, where it sits on the reservation as a comment. It reaches the right people, but it is not something the hotel has agreed to in advance.

Whether the hotel acts on it depends on the property and on how full it is that night. Some suppliers only forward a limited number of characters, and some drop anything they do not recognise, so short and plain wording works best. Requests that cost money, such as an extra bed or an early check in, are nearly always settled at the property itself and cannot be confirmed when the booking is made. If the difference really matters, it is safer to book the room type that already includes what the guest wants rather than to ask for it in a note.

To keep expectations sensible, make sure the wording beside that box on your portal says plainly that requests are passed on but cannot be promised, and check with your supplier which of those fields they actually forward before you tell your customers to depend on them.