Add Extra Input Fields In WooCommerce Registration Form

In the this example we will be adding a phone number field to the start of the form. It will be saved ad billing phone number. The code goes in functions.php file or in any other file that is connected to the functions.php file.

Adding input field to form

add_action('woocommerce_register_form_start', function(){
    woocommerce_form_field('billing_phone',[
        'type'=> 'tel',
        'required'=> true,
        'label'=> 'Phone',
    ], ( isset($_POST['billing_phone']) ? $_POST['billing_phone'] : '' ) );
});

Validate form input

add_action('woocommerce_register_post', function($username, $email, $errors){
    if( empty($_POST['billing_phone']) ){
        $errors->add('billing_phone_error', 'Please enter a Phone number.');
    }
}, 10, 3);

Save form input value

add_action('woocommerce_created_customer', function($customer_id){
    if( isset($_POST['billing_phone']) ){
        update_user_meta($customer_id, 'billing_phone', sanitize_text_field($_POST['billing_phone']));
    }
});

You can a more fields using this method and validate form values as per your need.