You are eligible to 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.
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 name replaced with your author's name): https://seekingalpha.com/author/author-name/research/reviews/news.
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