If it allows you to book online, the offer code should automatically be applied during the booking process, and after you've selected the room type, on the next screen, it should display the terms so you know you have the correct offer. If you are trying to book a comp/casino marketing offer, during the selection process, you may find that some hotels will let you book online and some will just mention a code and tell you to call. If those match up with what you expect, you have the right offer, there's no need to enter a code. If you are going through the booking process and have selected a room type, on the next screen, it should display the terms. I just looked and you're correct, I don t see any place to enter an offer code.
It's not clear to me how or what type of offer you are trying to book.