function jk_add_custom_sku() {
$args = array(
'label' => __( 'TC SKU', 'woocommerce' ),
'placeholder' => __( 'TC SKU', 'woocommerce' ),
'id' => 'tcsku',
'desc_tip' => true,
'description' => __( 'This SKU is for internal use only.', 'woocommerce' ),
);
woocommerce_wp_text_input( $args );
}
add_action( 'woocommerce_product_options_sku', 'jk_add_custom_sku' );