Subscription from website to automatically sync to iOS app

Hello,

I have a client who would like people to be able to subscribe (monthly or annually) on their website, then when they download the app they are automatically subscribed. At the moment, if they subscribe directly through the app, it renews automatically at the end of the subscription period, but if they've signed up through the website, the app isn't auto-renewed.

Is this possible?

Thanks,
Zach