App: Optimizer: Valid VAT & Address, how to assign to a form field?

Question:
I added the app Optimizer: Valid VAT & Address to validate VAT numbers with the European VAT check VIES on the checkout page. But I don’t understand how the check has to be applied to a specific form field. Now it always shows “invalid VAT number” on the top of the checkout page.

Product:
App: Optimizer: Valid VAT & Address

What are you trying to achieve:
Make the app validate VAT numbers on my checkout page

What have you already tried:
I created a custom form field called ‘VAT number’ where the customer can add their VAT number. Then I searched for ways to add the VAT check to this field.