Opt-in
Updated
Quick answer: A WhatsApp opt-in is a customer's permission for your business to message them on WhatsApp. WhatsApp's Business Messaging Policy requires opt-in before you send business-initiated messages. You can collect it on your website, at checkout, on a form, in store, or when the customer messages you first — and you should explain what kind of messages they will receive.
A customer's permission to receive your WhatsApp messages.
Always honour opt-out requests quickly. Messaging people who did not opt in leads to blocks and reports, which lower your quality rating.