You need to first register the number on WhatsApp Cloud API before you can begin messaging. Checkout their developer docs: https://developers.facebook.com/docs/whatsapp/cloud-api/reference/registration
Note, you will need to add the number to the Business Manager account and verify it in the process first.
Copy the Base64-encoded Certificate
A base64-encoded certificate string is generated for you and is used for validation during WhatsApp Business API client registration.
- Go to the Business Settings in your Facebook Business Manager.
- Click on WhatsApp Accounts under the Accounts menu, then select your WhatsApp Account from the WhatsApp Accounts list, if it isn’t already highlighted.
- Go to the Settings tab, and click the WhatsApp Manager button.
- Under the Phone Numbers tab, you should see your phone numbers listed. Click the View button of the phone number you want the certificate for.