There is a relevant Quora thread https://www.quora.com/What-are-the-best-sites-to-buy-App-Store-reviews-Android-and-or-iOS which features two dueling camps: one side extols the virtues of select Pay-for-Reviews sites, while the other takes the opposing view of Don't do that! - A sure way to get banned and/or banished; –and Yes, the Refill feature offered by some of those mentioned makes you wonder: What's the reason this is being offered, as in a "good reason" -?
Whenever the panel wraps up, you point out they could (only if they want to) leave a review for the app if they would like to.
You won’t be guaranteed positive reviews, but if they’re enthusiastic users and if you’ve been honest and forthright with them, they’ll at least leave thoughtful reviews.