Alter table add new field "ON UPDATE CURRENT_TIMESTAMP"

This is my table:

CREATE TABLE `megssage`(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(256) DEFAULT NULL,
  `time_create` datetime DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

I would like to add new column as time_updated with default value "ON UPDATE CURRENT_TIMESTAMP".

I tried this:

ALTER TABLE `megssage` 
   CHANGE `time_updated` `time_updated` TIMESTAMP NULL DEFAULT  ON UPDATE CURRENT_TIMESTAMP

But I am getting an error. Can someone help me?