WooCommerce - Add a new Custom Checkout Field

functions.php

add_action( 'woocommerce_after_order_notes', 'xa_new_custom_checkout_field' );
function xa_new_custom_checkout_field( $checkout ) {

    echo '<div id="xa_new_custom_checkout_field"><h2>' . __('SSN') . '</h2>';

    woocommerce_form_field( 'new_field_ssn', array(
        'type'          => 'text',
        'class'         => array('ssn-field-class form-row-wide'),
        'label'         => __('SSN Number'),
        'placeholder'   => __('Enter SSN'),
        ), $checkout->get_value( 'new_field_ssn' ));

    echo '</div>';
}