One trick might be to just put a 10% higher price within the app, and lower off app. Consumers will think it's a mistake and spread the word. You can also send them an email telling them that they get a discount if they subscribe now.
On the other hand, some people prefer to pay within app. I don't trust many subscription services to be responsible with my credit card information, but Apple/Google have proven to have consumer protection. So you should probably do both.
In your app, don't put a link or any text that would lead or let user know they can make payment outside of App Store.
Set up an external website that registers/login user and process user payment there. Then in your app, require user login and then your app can communicate with the backend to check if the user is subscribed.