As we warned about a few months back, Apple is now requiring apps to use the Apple-provided API to ask users for reviews and anything else will be rejected.

From section 1.1.7 of the updated 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.

If you’re using a custom review prompt in your app, expect to get flagged for this soon and make arrangements to use Apple’s official API going forward.