You can leave a review after being a member for at least 15 days. Once that is the case, you'll see a pop-up when you log into your service.
For the first 15 days you're in the service; you will not have the ability to leave a review as we feel it's important to really familiarize yourself with a service before telling others about it.
To do this proactively, you can click on tools, and select 'write a review' from the list:
You can also leave a review by going to the URL for your author, which will look like this (with the author's name replaced with your author's name): https://seekingalpha.com/author/author-name/research/reviews/news.