You first must create the sequence you want to use to add the identity value for the column. Then replace the values in highlighted below appropriately for your environment.
CREATE OR REPLACE TRIGGER {SCHEMA.TRIGGER_NAME}
BEFORE INSERT ON {SCHEMA.TABLE_NAME} FOR EACH ROW
BEGIN
IF :NEW.{COLUMN_NAME} IS NULL
THEN
SELECT {SCHEMA.SEQUENCE_NAME}.NEXTVAL INTO :NEW.{COLUMN_NAME} FROM DUAL;
END IF;
END;