Supplier Registration Page customization to invoke external API

Summary:

We have a requirement to invoke the external API from Supplier Registration Redwood page to validate the Tax registration number on tabbing out of the Tax Registration Number field or on click of save button.

I don't see any Action Chains when I edit the page in VBS, I could find the the event "submitActionEvent" but there is no event listener or Action Chain built on it. If I build the custom Action chain I can raise the event but I won't be able to stop the save/submit.

How this requirement can be achieved and how can I invoke the External API and control the flow not to save/submit on invalid tax registration number response.