Add a field to Woocommerce registration form and in admin edit user

The following will add a "billing account number" field in customer registration as a required field (validation) and will also display it:

• In My account > Edit account section
• In Admin User edit pages on billing fields section

The code:

// Display a field in Registration / Edit accountadd_action( 'woocommerce_register_form_start', 'display_account_registration_field' );add_action( 'woocommerce_edit_account_form_start', 'display_account_registration_field' );function display_account_registration_field() {    $user  = wp_get_current_user();    $value = isset($_POST['billing_account_number']) ? esc_attr($_POST['billing_account_number']) : $user->billing_account_number;    ?>    <p class="woocommerce-form-row woocommerce-form-row--wide form-row form-row-wide">    <label for="reg_billing_account_number"><?php _e( 'Ship to/ Account number', 'woocommerce' ); ?> <span class="required">*</span></label>    <input type="text" maxlength="6" class="input-text" name="billing_account_number" id="reg_billing_account_number" value="<?php echo $value ?>" />    </p>    <div class="clear"></div>    <?php}// registration Field validationadd_filter( 'woocommerce_registration_errors', 'account_registration_field_validation', 10, 3 );function account_registration_field_validation( $errors, $username, $email ) {    if ( isset( $_POST['billing_account_number'] ) && empty( $_POST['billing_account_number'] ) ) {        $errors->add( 'billing_account_number_error', __( '<strong>Error</strong>: account number is required!', 'woocommerce' ) );    }    return $errors;}// Save registration Field valueadd_action( 'woocommerce_created_customer', 'save_account_registration_field' );function save_account_registration_field( $customer_id ) {    if ( isset( $_POST['billing_account_number'] ) ) {        update_user_meta( $customer_id, 'billing_account_number', sanitize_text_field( $_POST['billing_account_number'] ) );    }}// Save Field value in Edit accountadd_action( 'woocommerce_save_account_details', 'save_my_account_billing_account_number', 10, 1 );function save_my_account_billing_account_number( $user_id ) {    if( isset( $_POST['billing_account_number'] ) )        update_user_meta( $user_id, 'billing_account_number', sanitize_text_field( $_POST['billing_account_number'] ) );}// Display field in admin user billing fields sectionadd_filter( 'woocommerce_customer_meta_fields', 'admin_user_custom_billing_field', 10, 1 );function admin_user_custom_billing_field( $args ) {    $args['billing']['fields']['billing_account_number'] = array(        'label'         => __( 'Ship to/ Account number', 'woocommerce' ),        'description'   => '',        'custom_attributes'   => array('maxlength' => 6),    );    return $args;}

Code goes in function.php file of your active child theme (active theme). Tested and works.