@type t() :: %Paypal.Subscription.Create{
  application_context: map() | nil,
  custom_id: String.t() | nil,
  plan: map() | nil,
  plan_id: String.t() | nil,
  quantity: String.t() | nil,
  shipping_amount: Paypal.Common.CurrencyValue.t() | nil,
  start_time: String.t() | nil,
  subscriber: Paypal.Subscription.Subscriber.t() | nil
}

Fields for creating a subscription:

• plan_id - The billing plan ID (required).
• start_time - Start time for the subscription (ISO 8601 string or datetime).
• quantity - Subscription quantity.
• shipping_amount - Shipping amount struct or map.
• subscriber - Subscriber struct or map.
• application_context - Map containing return_url, cancel_url, brand_name, locale, etc.
• custom_id - Custom ID for tracking.
• plan - Custom plan override if applicable.