To Add Payment method for Whatsapp:

1. Go to https://business.facebook.com/billing_hub

2. Select Correct Business Manager in which you have Set up Whatsapp Number.
   

1. Click on Payment methods on the Right Menu
2. Click on Whatsapp Business Accounts and from the Dropdown select correct Whatsapp Business Account
   

1. Add Payment Method Debit/Credit Card
   

1. Continue and Complete Process.
   
2. Once Payment is Completed you will see the below screen.
   

Once Payment Method is enabled you will be Billed automatically via Facebook.

To View Your Total Due Payment regularly or to download payment receipts you can always navigate back to same URL.

Payment Method not Accepted/ Errors.