This is true, I did just go through this (n the past week) and, as well, am completely blank as to how to do it. However, Pierre is a superstar, so if you just go to the Smartbnb website, and look at the information, you should be able to figure it out. Once I set it up, I received an email from them, and when I responded to it, asking for help, Pierre immediately responded and made sure everything was set up to for the review to go out at the very last minute (he helped me out by telling me what had to be changed so that it would work).
I was also confused about how much time I actually had left, but like @Astaire mentioned, once you get to the last 24 hours of the time you can still write a review, it does do the countdown. @konacoconutz told me to check on the Dashboard which is where I could see it, as well.