To send an app review request for iOS 10.2 and earlier or Google Play (Android app store), create a new in-app message and select the Confirm template.
Note: If you want to send a request for iOS 10.3 and up, see here.
Set up the message's text with your custom rating request, and change the text of the Accept and Cancel buttons to fit your rating message.
Set the Accept action to "Open URL" and paste in the link to your app's review page in the App Store. The cancel action should be "No action," or you can chain to another message or link asking for additional feedback.
For an example of an effective app review request message flow, see How to get more five-star app reviews.