Add field to WooCommerce product information panel

functions.php

/**
 * Add field to product options
 *
 * @return	void
 */
function projectnamespace_add_product_fields() {
	
	woocommerce_wp_checkbox( 
		array( 
			'id'    => '_projectnamespace_enquiry_only', 
			'label' => __( 'Enquiry Only', 'woocommerce' ), 
		)
	);

}
add_action( 'woocommerce_product_options_pricing', 'projectnamespace_add_product_fields' );


/**
 * Save field to product post meta
 *
 * @param	int		$post_id
 * @return	void
 */
function projectnamespace_save_product_fields( $post_id ) {
	
	$fields = array( '_projectnamespace_enquiry_only' );
	foreach ( $fields as $field )
		update_post_meta( $post_id, $field, esc_attr( isset( $_POST[ $field ] ) ? $_POST[ $field ] : '' ) );
		
}
add_action( 'woocommerce_process_product_meta', 'projectnamespace_save_product_fields' );