Updates fields for an existing subscriber.

form_id (optional)

If provided, the update runs through that form's pipeline (field permissions, automations, etc.).
If omitted or 0, a list-level update is performed.

send_notice

Set send_notice: true to trigger the form's admin notification email after the update.

Multipart / file upload

Also accepts Content-Type: multipart/form-data for lists with file-upload fields.

Error codes