Stripe-Elixir How to get “payment source”

I am trying to create a customer that subscribes to a plan.

defmodule StripeTestWeb.PaymentController do
use StripeTestWeb, :controller
use Stripe.API

def index(conn, %{"stripeEmail" => email} = _params) do
{:ok, customer_data} = Stripe.Customer.create(%{email: email})

Stripe.Subscription.create(%{
customer: customer_data["id"],
items: [%{plan: "plan_D9JehUOiyPGtgp"}]
})

render(conn, "index.html")
end
end


The customer is created but the request returns an error when trying to create the subscription. The error is:

{
"error": {
"code": "resource_missing",
"doc_url": "https://stripe.com/docs/error-codes/resource-missing",
"message": "This customer has no attached payment source",
"type": "invalid_request_error"
}
}


I don't know what the API expects me to send as "payment source" nor am I clear if this is supposed to be sent when creating the customer or when creating the subscription.

I am using the JavaScript embedded code to create the payment pop up:

<form action="payment" method="POST">
<input type="hidden" name="_csrf_token" value="<%= Plug.CSRFProtection.get_csrf_token()%>">

https://checkout.stripe.com/checkout.js

</form>


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