Creates a new signup, data-collection, or order form within the given subscriber list and
immediately generates its HTML source, so it is publicly accessible via formUrl.

Form method

Subscribe and update forms

Double opt-in

Fields on the form

Use the optional fields array to place list fields on the form at creation time.

Each element references a list field by its technical name (fieldName) and may override
the display label, mandatory flag, validation message, and position.

Subscribe forms (method: "subscribe")

• The email field is always required on the form.
• If fields is omitted, the form is created with the email field only.
• If fields is provided but does not contain "email", the email field is
  automatically prepended with isMandatory: true.
• All fieldName values must exist on the subscriber list.

Update forms (method: "update")

• fields is required and must contain at least one element.
• All fieldName values must exist on the subscriber list.

Field options (radio, select and similar types)

List fields of type radio, horizontalradio, select, or textmulti require an
options array. Each option defines the stored value, the display label, and whether it is
pre-selected by default.

• options is required for the above field types and must contain at least one element.
• options is not allowed for other field types (e.g. text, textarea) — providing it
  returns a 400 error.
• Each option must have a value key (may be an empty string) and a non-empty text.

Order forms (method: "order")

Order forms allow subscribers to purchase one or more products. They require:

• shippingMethodIds (required): one or more shipping method IDs that must belong to your account.
  Use GET /shipping-method to list available shipping methods.
• orderType (required): controls the product selection behaviour on the form.

  Value	Behaviour
  1	Fixed product — exactly one product, no quantity control
  2	Product selector — customer selects from a list
  3	Quantity-based — customer enters a quantity per product
  4	Subscription / recurring

• products (required): list of products to offer on the form.
  Each entry must have either a productId (integer) or a sku (string) to identify the product.
  • If sku is given instead of productId, the system resolves it to the matching product in your account.
  • orderType: 1 allows exactly one product.

Order form product fields

Order bump (checkbox upsell)

A product with orderBump: true is not part of the main product selection: it is shown
as a checkbox offer at the end of the order form, in a box styled by bumpStyle.

• Not available with orderType: 1.
• At least one product without orderBump is required.
• Cannot be combined with defaultChecked (bump products are never pre-selected).
• A bump product can be listed only once on the form.
• bumpStyle properties (all optional): borderColor, borderWidth (0-10 px),
  borderStyle, backgroundColor, padding, showPrice (show / hide), align
  (left / center) and imageUrl (e.g. an animated GIF shown before the checkbox).
• Bump products are rendered by the default order form template only.

What happens after creation

1. The form record is inserted in the database.
2. For subscribe/update forms: the specified fields (and their options) are added to the form.
   For order forms: the selected products are linked to the form.
3. The form HTML is generated and uploaded to S3 at:
   newsletter/letter/nl{listId}/ns{formId}/subscribe.html
4. The public formUrl is returned so you can embed or link to it immediately.

You can manage form fields later via the following endpoints:

• Get form fields and list fields —
  GET /newsletter/{newsletterId}/form/{formId}/fields
• Add fields to a form —
  POST /newsletter/{newsletterId}/form/{formId}/fields
• Remove a field from a form —
  DELETE /newsletter/{newsletterId}/form/{formId}/field/{fieldName}