Three endpoints cover almost every sync scenario:
Call this from your signup handler, checkout flow, or form webhook (e.g. a Wix or WordPress form):
Use tags to record where the contact came from — you can segment and trigger automations on tags later.
Idempotent sync: upsert
If the same email may be submitted more than once, use upsert instead — it creates the contact if new, updates it if it exists, and won’t fail on duplicates.
Bulk: batch create
For imports, send up to a full batch in one call with contacts (array) and list_id. For very large one-time migrations, consider the CSV import wizard in the app instead — it includes field mapping and validation.
Custom fields
Contacts accept a custom_fields object keyed by your account’s custom-field IDs. List your fields via the app under Audience → Custom fields, or see Custom Properties to manage them via API. 
