To send your customers a link to review a product, add "?ref=review" to the end of your product page URL. The review form will open above the product page when it loads.
Example:
Product page URL: http://www.mystore.com/products/blue-shirt
Product review URL: http://www.mystore.com/products/blue-shirt?ref=review
Note: The ?ref=review has to be added directly after the product handle, and will not work if added after additional query strings, such as product variants.