DBMS_SESSION.SET_IDENTIFIER

begin
   dbms_session.set_identifier('tq84');
end;
/

select *
from
   v$session
where
   client_identifier = 'tq84';

select
   sys_context('userenv', 'client_identifier')
from
   dual;

begin
   dbms_monitor.client_id_trace_enable(
      client_id => 'tq84'
   );
end;
/