If you didn’t set up an account immediately after purchasing a subscription in-app, you can create a linked account by going to:
account > existing subscriptions > create account
Once you’ve created a linked account from the app, you can use those details to access the website.
If you already have an account
If you've previously created or had an account outside the app, log into the app with your existing details and then select create account, you will then see a message asking you to link your account. Once completed, your app store subscription will work on the website.