Short answer
HubSpot's KB: after connecting WhatsApp, you must get explicit consent. Contacts opt in with keywords in a thread, or you opt them in manually with verifiable consent. Updating opt-in via workflows requires a connected WhatsApp channel.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Keep a record of where each manual opt-in came from.
3. Related
WhatsApp statuses and WhatsApp in help desk.