Referencia: 
https://firebirdsql.org/file/documentation/release_notes/html/en/3_0/rnfb30-psql-ddltriggers.html 

create or alter trigger TRG_PADRAO_CRIA_TAB
active after create table position 0
as
declare variable vs_Tabela varchar(50);
begin
   
   vs_Tabela = rdb$get_context('DDL_TRIGGER', 'OBJECT_NAME');

   -- Cria a PK
   execute statement 'alter table ' || :vs_Tabela  || ' add constraint PK_' || :vs_Tabela || ' primary key (ID)';

   -- Cria o generator
   execute statement 'create sequence GEN_' || :vs_Tabela  || ' start with 0 increment by 1';

   -- Cria a trigger que alimenta o generator
   execute statement
'create or alter trigger TRG_' || :vs_Tabela || '_BI0 for ' || :vs_Tabela ||
' active before insert position 0
as
begin

   /*
      Sobre...: Novo registro
      Revisao.: 1
   */

   if ((new.ID is null) or (new.ID<=0)) then
      new.ID = gen_id(GEN_'|| :vs_Tabela || ',1);

end';

end