curl --request POST \
  --url https://app.pennylane.com/api/external/v2/gocardless_mandates/mail_requests \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "customer_id": 42,
  "email": {
    "subject": "Please authorize your direct debit mandate",
    "body": "Please click the link to authorize your direct debit mandate.",
    "recipients": [
      "customer@example.com"
    ]
  }
}'

This response does not have an example.