SQL> create table test(
  2                    id number(*,0) GENERATED by default as identity,
  3                    city varchar2(100),
  4                    HT_ID VARCHAR2(100 CHAR) generated always as (id || 'HT')
  5                   )
  6  /

Table created.

SQL> insert into test (city)
  2  values ('saratov')
  3  /

1 row created.

SQL> column city format a10
SQL> column ht_id format a10
SQL> select  *
  2    from  test
  3  /

        ID CITY       HT_ID
---------- ---------- ----------
         1 saratov    1HT

SQL>