In the API Reference (https://api.brevo.com/v3/emailCampaigns) when creating the Email Campaign you can send in the body of the request a property called previewText (Preview text or preheader of the email campaign). There is no such property in the definition of CreateEmailCampaign.
Originally posted by @Innsai in https://github.com/sendinblue/APIv3-python-library/issues/66#issuecomment-1702562826