Can I book a hotel room for tonight through a hotel booking API?

Yes, in most cases you can book a room for the same night through a hotel booking API. Same day availability is a normal part of hotel distribution, and the search will simply return the properties that still have rooms open for that night. The list your customer sees will usually be shorter than for a stay further ahead, because many properties are already full by then or have closed their online sales for the day.

The part worth understanding is that every supplier sets its own cut off time for same day arrivals. Some keep selling until late in the evening, some stop in the afternoon, and some pass the booking to the hotel to confirm instead of confirming it straight away. That is decided by the supplier and by the individual property, not by your portal, so the only reliable guide is what comes back in the live search at the moment your customer is actually looking.

Last minute rooms also tend to be non refundable, or to carry very tight cancellation rules, so it is worth showing those rules plainly on the booking page before your customer pays. Confirmation matters more than usual here too, because your customer may be walking up to the front desk within hours of paying, and you want the hotel reference in their hands before they travel.

If selling last minute stays is important to you, run a few test searches for tonight at different times of the day in the destinations you sell most, and ask your supplier what their cut off time and confirmation behaviour is for same day arrivals.