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_in  text
                )
          returns text
         language sql strict
as $$
    select e.enumlabel::text
      from pg_type t
      join pg_enum e on t.oid = e.enumtypid
      join pg_catalog.pg_namespace n on n.oid = t.typnamespace
     where t.typname = lower(relation_name_in)
     order by random()
     limit 1;
$$ ;

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

select * from testtable;