You can send out ARCS to solicit reviews before you launch your book. However, they cannot leave reviews on Amazon until some version of that book is live and for sale on Amazon.
So... Usually, people release their paperback version first, and direct arc reviewers to that link to leave reviews. Then, then when the ebook goes live, Amazon links the paperback and ebook versions, and all those early reviews appear on both editions.
I hope that helps.
*Or*
You can do the legwork of gathering "Editorial Reviews" from other authors and organizations/publications, and post those in the editorial reviews section of your product page ahead of launch.