A custom field for a contact.
Create or Update a Contact
While custom fields comes with their own benefits you can add/remove data using API even if you haven't created the custom field....To update a contact's custom fields, include the custom field data in the JSON body of your POST:
While not required, we recommend custom field key is lower case, contain no spaces, and are limited to alphanumeric characters....This ensures that if the custom field were used for API access, such as personalization, the customer field could be accessed.