How to set default value for datetime type in MySQL

CREATE TABLE `app_info1` ( 
 `id` bigint(21) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Primary key ID', 
 `app_id` varchar(30) DEFAULT NULL COMMENT 'Application code', 
 `app_name` varchar(30) DEFAULT NULL COMMENT 'Application name', 
 `package_name` varchar(100) DEFAULT NULL COMMENT 'Package name', 
 `version` int(11) DEFAULT NULL COMMENT 'Version number', 
 `version_name` varchar(20) DEFAULT NULL COMMENT 'Version name', 
 `icon_url` varchar(512) DEFAULT NULL COMMENT 'Icon address', 
 `download_url` varchar(512) DEFAULT NULL COMMENT 'Download URL', 
 `summary` varchar(512) DEFAULT NULL COMMENT 'Summary', 
 `desc` varchar(512) DEFAULT NULL COMMENT 'Description', 
 `app_status` int(4) DEFAULT '0' COMMENT 'Status 0: Available, 1: Deleted', 
 `create_time` datetime DEFAULT CURRENT_TIMESTAMP COMMENT 'Creation time', 
 `create_user` varchar(30) DEFAULT NULL COMMENT 'creator', 
 `modify_time` datetime DEFAULT CURRENT_TIMESTAMP COMMENT 'Modification time', 
 `modify_user` varchar(30) DEFAULT NULL COMMENT 'Modifier', 
 PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

alter table ts_role 
MODIFY column create_tm datetime default CURRENT_TIMESTAMP COMMENT 'Creation time'