It is actually helpful to do this in writing.

As this is a departure from the US, US law affords you a full refund within 7 days of your request when any segment is cancelled (or significantly changed). Simply send the email with the requested information and list as a reason, "because the carrier cancelled a flight."

In the event that your refund is not initiated within 7 days or your request and presuming that you paid with a card issued by a US financial institution, you have an easy chargeback remedy which you are equally easily able to document (copy of notice of cancellation + your email requesting a refund).