gistfile1.txt

// Display Fields
add_action( 'woocommerce_product_options_general_product_data', 'woo_add_custom_general_fields' );

// Save Fields
add_action( 'woocommerce_process_product_meta', 'woo_add_custom_general_fields_save' );

function woo_add_custom_general_fields() {

	global $woocommerce, $post;
	
	// Text Field
	woocommerce_wp_textarea_input(
		array(
		'id'          => '_textarea',
		'label'       => __( 'My Textarea', 'woocommerce' ),
		'placeholder' => '',
		'desc_tip'    => 'true',
		'description' => __( 'Enter the custom value here.', 'woocommerce' )
	)
	);
}
function woo_add_custom_general_fields_save( $post_id ){

	// Textarea
	$woocommerce_textarea = $_POST['_textarea'];
	if( !empty( $woocommerce_textarea ) )
	update_post_meta( $post_id, '_textarea', esc_html( $woocommerce_textarea ) );

}