How to add payment method in Meta
To connect to the whatsApp business API you'll need to setup your payment method. This is how to set it up when you already work with the WhatsApp Business App.
1 - Login:
Visit business.facebook.com with your credentials
Login needs to be with facebook for the API connection, if you only have instagram logins connect with those, we'll add a facebook logins at a later stage.

If don't have Meta Business Portfolio yet create one following their suggested flow.
⚠️ Please don't create a fake facebook profile to set this up. Facebook easily flag those profiles as scam and it might shortly after creation block your portfolio. If you'd like to add your company email as login option instead of your old facebook logins use this doc: How to add your company email as a Facebook login.
2 - Business Portfolio
Select from the dropdown the portfolio that belongs to the company we're connecting to the WhatsApp API
Once selected click the "settings gear".

3 - Billing and payment
Scroll down and click "Billing and payment" this will open a new tab:

Go to messaging accounts and click on your whatsApp account.

4: Once you click on your account you'll get a similar view like the below. Next:
Add your company legal information by clicking the button "edit" and click "add payment method":

5- Payment Method
Be sure to set the payment method as default:

This will complete the process.