It sort of is against the rules (disallow custom review prompts) but it doesn't seem to get enforced as far as I can tell. Even top apps like YouTube do this.

https://developer.apple.com/app-store/review/guidelines/

> Use the provided API to prompt users to review your app; this functionality allows customers to provide an App Store rating and review without the inconvenience of leaving your app, and we will disallow custom review prompts.