MariaDB [test]> CREATE TABLE definer_current_user (f1 INTEGER, trigger_user VARCHAR(100)) ENGINE=InnoDB;

Query OK, 0 rows affected (0.10 sec)

MariaDB [test]> CREATE DEFINER=current_user TRIGGER definer_current_user BEFORE INSERT ON definer_current_user FOR EACH ROW SET NEW.trigger_user = CURRENT_USER();

Query OK, 0 rows affected (0.06 sec)

MariaDB [test]> show tables;

+----------------------+

| Tables_in_test       |

+----------------------+

| definer_current_user |

+----------------------+

1 row in set (0.00 sec)

MariaDB [test]> INSERT INTO definer_current_user (f1) VALUES (1);

ERROR 1449 (HY000): The user specified as a definer ('*current_user'@'%') does not exist