create type  myenum
as enum (
   'one',
   'two',
   'three',
   'four'
);

create table testtable (
  id                       integer PRIMARY KEY,
  name myenum
);


create or replace function random_enum(relation_name anyelement, out result anyenum)
returns anyenum as $$
begin
  execute format(
    $sql$
      SELECT elem
      FROM unnest(enum_range(NULL::%1$I)) AS elem
      ORDER BY random()
      LIMIT 1;
    $sql$,
    pg_typeof(relation_name)
  ) into result;
  return;
end;
$$ language plpgsql;

insert into testtable (id, name) values (1, random_enum(null::myenum));

select * from testtable;