Identifying the user on 'charge:confirmed' when using Coinbase Checkout

[duplxey]

How can you figure out (in the webhook on charge:confirmed) who purchased a product when using Coinbase Checkout?

As far as I know there's no way to attach metadata when creating a Checkout as you would with Charges:

 metadata: {
    customer_id: 1,
    customer_username: "JohnDoe"
}