Research by Yotpo of 200K stores and 163M orders discovered the impact of various elements in the email subject line on the number of reviews obtained:

1) Exclamation point (!)

 

2) Question mark (?)

 

3) Store name

 

4) UPPERCASE

 

5) Incentives (coupon, discount, sale)

 

This article on Jilt contains 3 samples of Product Review Request emails (towards the bottom of the article):

https://jilt.com/blog/ask-for-product-reviews/

Reviews.io has several example emails also:

https://blog.reviews.io/5-awesome-review-collection-email-examples

A few more examples and guidelines here:

https://www.campaignmonitor.com/blog/email-marketing/2019/08/8-best-practices-to-encourage-customer-reviews-through-email/