Your Formatting is off, remember DELIMITER $$
statements are required, as it is no different than creating a stored procedure.
Check the documentation any time you run into issues like this.
Scroll about half way down the documents to see the example.
But, This is what your looking for:
DELIMITER $$
CREATE TRIGGER tg_newuser_insert
BEFORE UPDATE ON tbl_newuser
FOR EACH ROW
BEGIN
SET NEW.id = CONCAT('YTUM', LPAD(LAST_INSERT_ID(), 8, '00000'));
END $$
DELIMITER ;
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