@type t() :: %PolarExpress.Schemas.SubscriptionCreateExternalCustomer{
  external_customer_id: String.t() | nil,
  metadata:
    %{
      required(String.t()) => String.t() | integer() | float() | boolean() | nil
    }
    | nil,
  product_id: String.t() | nil
}

• external_customer_id - The ID of the customer in your system to create the subscription for. It must already exist in Polar.
• metadata - Key-value object allowing you to store additional information.

The key must be a string with a maximum length of 40 characters. The value must be either:

• A string with a maximum length of 500 characters
• An integer
• A floating-point number
• A boolean

You can store up to 50 key-value pairs.

• product_id - The ID of the recurring product to subscribe to. Must be a free product, otherwise the customer should go through a checkout flow. Format: uuid4.