Before you can message a customer using WhatsApp, you must collect their opt-In (i.e. consent) to comply with both WhatsApp requirements and GDPR regulations. This can be done via Online, SMS, Email etc.
Please refer to the WhatsApp Opt-in Examples document for various opt-in examples.
Learn how you can send WhatsApp messages to your customers easily and securely using the Sinch WhatsApp Business API.