To execute multiple commands like a function (with PgAdmin), you have to wrap the commands in a DO block:
DO
$do$
DECLARE
_my_text text = 'HELO';
BEGIN
RAISE NOTICE '_my_text: %', (_my_text::text); --in this specific case ::text is unnecessary because _my_text is already a text
DECLARE t_record RECORD;
BEGIN
FOR t_record IN
SELECT * FROM public.comp
LOOP
RAISE INFO 'Row in myTable: %', t_record::text;
END LOOP;
END;
END
$do$;