Register page validation

Hello,
I need to write some custom validation rule for input field in registration page (for example for billing address).
as I understand I need to connect to " saveRegisterAction" in Register controller, but couldn’t find how.

Please advice.

Thank you.

We provide a plugin that does the same for countries: Shipping Restriction | Shipping | Integration | Extensions | Shopware Community Store

You can download it for free and it is open source, perhaps this can guide you to your own solution.