curl --request POST \
  --url http://localhost:8080/users/{user_id}/mandate/register \
  --header 'Content-Type: application/json' \
  --data '
{
  "amount": 123,
  "account_id": "<string>"
}
'

{
  "amount": 123,
  "created_at": "<string>",
  "customer_id": "<string>",
  "frequency": "as_presented",
  "id": "<string>",
  "last_modified_at": "<string>",
  "mandate_status": "pending",
  "max_amount": 123,
  "order_id": "<string>",
  "payload": "<unknown>",
  "user_id": "<string>",
  "account_id": "<string>"
}