Hi @wax
I’m afraid WooCommerce has a custom registration form on the checkout page. To modify it, please take a look at this page: https://docs.woothemes.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/. To update the custom field value in the database, you can use the update_field() function. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/update_field/.
I hope this helps. If you have any other questions, feel free to ask me again 🙂
