How do I cancel a subscription that has just been added on my account?

Review recent purchases at https://reportaproblem.apple.com/ or check your purchase history in iTunes and view your subscriptions. If you are the organizer of a family share check for purchases by other members.

See also If you see an unfamiliar iTunes Store or App Store charge on your bank, credit card, or debit statement - Apple Support and About payment card authorization holds in the iTunes Store - Apple Support.

If you cannot identify the payment then perhaps you are looking at a phishing email.

If there really have been purchases that you didn't authorize, or you've entered your Apple ID credentials at a fake Apple portal, see If you think your Apple ID has been compromised - Apple Support, change your Apple ID password, and if possible turn on Two-factor authentication for Apple ID - Apple Support.

See also Use Restrictions to prevent purchasing on your iPhone, iPad, or iPod touch - Apple Support.

• If there is no evidence of a charge on either your iTunes or bank/credit card accounts this is likely to be a phishing exercise which you should ignore.
  
• If there is a charge shown under your iTunes store account and it relates to misuse of your Apple ID then you need to start a conversation with iTunes Store support to get it cancelled and refunded, change your password, and enable two-factor authentication if at all possible.
  
• If it isn't connected to your own Apple ID but funds have gone from your bank or credit card account that purport to be from iTunes then the account has been compromised and you need to deal directly with your financial institution.

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