Add items to a list

Adds items to the specified list. Values are normalized (lowercased and trimmed) and validated against the list's type — domain lists accept domain names, tld lists accept top-level domains, and address lists accept full email addresses. Duplicate additions are silently ignored.

The response returns the updated List object with a refreshed items_count. You can submit up to 1000 items per request, and each item value can be at most 500 characters. Values that exceed this length return a 400.