For what platform is this? There is plugins atleast for iOS and Android and maybe a plugin that will give you the rating functionality for both platforms. The 15th use time functionality you have to implement yourself. Just keep a counter in save data how many times the user has launched the app and when user launched 15th time you call your rate functionality to activate.
Fast googling found this: https://github.com/nicklockwood/iRate for ios and
this for android: https://github.com/Kopfgeldjaeger/RateMeMaybe .
I haven’t used the above plugins and there is probably much more to be found if you google something like “rate my app plugin”.