At first I was a little flummoxed by your question because I knew I'd asked questions on Booking.com before.
I took little look and the question asking options appear below the apartment details on my tablet.
Here's the first one: Questions you may have
Find quick answers or ask questions to the property.
When you click on this you'll see questions others have asked with the answers. And there's an option to ask a question yourself.
Below that is an option to do a live chat and ask questions, but I think that's with a Booking.com person.
Both these options in this location make sense to me because by that point you should have taken in the major information about the place.
Scroll farther down to see things like Apartment Policies and Important Information. These are the links to find out admin things and the fine print that can trip renters up if they don't read them.
There's also a 2nd chance to ask questions to the property there. This makes sense to me because it's close to that fine print, so you don't have to scroll back and forth much to formulate your question.
I have not messed around with this on my desktop computer. Things may be in different places on the screen there. I know this RS website sets itself up very differently on my PC than on my tablet. I did check my phone and it sets up the same as my tablet for both Booking.com and the RS website.