I need to add a field to the user registration form and then validate it before I add a new user to my WordPress

I know how to hook into the registration form via add_filter('register_form', 'my_function');

But how do I hook into the registration process before the user registers (hence I do not want add_action('user_register','my_other_function');).

Thanks for helping me.

I need to add a field to the user registration form and then validate it before I add a new user to my WordPress

I know how to hook into the registration form via add_filter('register_form', 'my_function');

But how do I hook into the registration process before the user registers? Hence I do not want add_action('user_register','my_other_function');.