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For PayPal you can use API calls to activate a subscription from a server.
This gives you reliable server-side notification that a subscription is started.
To be notified of actual payment transactions on the subscription, which typically start the following day and repeat on its cycle, you need a webhook listener for the event PAYMENT.SALE.COMPLETED
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You could also try to use the ExtensionPay service which gives you a simple API to take payments in your extension.
Source:stackexchange.com