Call this from your backend when a customer wants to connect WhatsApp. Send an allowed HTTPS returnUrl and optional opaque metadata that identifies the customer in your system. Set connectionMode to coexistence to keep an existing WhatsApp Business App number active while adding Cloud API access. Coexistence requires the dedicated Meta configuration and cannot be used as a number-migration destination. Redirect the customer to hostedUrl. The session expires after 15 minutes and no managed user is created until the hosted Meta Embedded Signup succeeds.