SQL> create table test (col varchar2(20));

Table created.

SQL> create or replace trigger trg_test_c
  2    before insert on test
  3    for each row
  4  begin
  5    :new.col := 'C: ' || :new.col;
  6  end;
  7  /

Trigger created.

SQL> insert into test (col) values ('1234');

1 row created.

SQL> select * From test;

COL
--------------------
C: 1234

SQL>

create or replace trigger trg_Reservation_CN
  2    before insert on Reservation
  3    for each row
  4  begin
  5    :new.confirmationNumber := 'CN: ' || :new.confirmationNumber;
  6  end;
  7  /