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