FB Page <> WhatsApp API Linking

You can add a button to your Facebook Page that clicks to WhatsApp.
You can add an action button to your Facebook Page that clicks to your WhatsApp Business phone number. To do this, you must be an admin, editor, moderator or advertiser for the Facebook Page.
1) On Facebook Page click + Add a Button
2) Choose Send WhatsApp Message
3) Enter your WhatsApp phone number
If you already own a WABA and have registered a WA number, that number will pre-populate the field.
4) Enter the verification code
That's it! Clients can now start chatting with you directly from your Facebook Page.
5) If your Facebook Page and WhatsApp phone number sit with different owners / Facebook Business Manager Accounts (e.g. an ad agency), 360dialog must approve the request. Please reach out to use via Support Ticket and we will approve the request for you.

Facebook Documentation