Create CheckoutSession

curl --request POST \
  --url https://app.flowglad.com/api/v1/checkout-sessions \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '{
  "checkoutSession": {
    "customerExternalId": "<string>",
    "successUrl": "<string>",
    "cancelUrl": "<string>",
    "outputMetadata": {},
    "outputName": "<string>",
    "type": "product",
    "priceId": "<string>",
    "quantity": 123
  }
}'

{
  "checkoutSession": {
    "id": "<string>",
    "createdAt": "2023-11-07T05:31:56Z",
    "updatedAt": "2023-11-07T05:31:56Z",
    "livemode": true,
    "status": "open",
    "billingAddress": {
      "name": "<string>",
      "firstName": "<string>",
      "lastName": "<string>",
      "email": "jsmith@example.com",
      "address": {
        "name": "<string>",
        "line1": "<string>",
        "line2": "<string>",
        "city": "<string>",
        "state": "<string>",
        "postal_code": "<string>",
        "country": "<string>"
      },
      "phone": "<string>"
    },
    "priceId": "<string>",
    "purchaseId": "<string>",
    "invoiceId": "<string>",
    "quantity": 123,
    "organizationId": "<string>",
    "customerName": "<string>",
    "customerEmail": "<string>",
    "customerId": "<string>",
    "paymentMethodType": "card",
    "discountId": "<string>",
    "successUrl": "<string>",
    "cancelUrl": "<string>",
    "type": "purchase",
    "outputMetadata": {},
    "outputName": "<string>",
    "targetSubscriptionId": "null"
  },
  "url": "<string>"
}