Problem with trigger function

I am trying to implement a trigger in a PostgreSQL-9.6.17 database:

CREATE OR REPLACE FUNCTION customer_num_informix()
  RETURNS trigger AS $$
BEGIN
  -- An Aubit/Informix 4GL INSERT statement passes the value 0 to Informix
  -- DBMS for columns that have the SERIAL data type.  Informix will then
  -- use the incremented serial number in place of 0. PostgreSQL instead
  -- will simply take the value 0 and replace the incremented serial number.
  -- This trigger function emulates the Informix DBMS behaviour.
  --
  -- The NEW variable contains the data for the row to be INSERTed or
  -- UPDATEd. It, and its complement OLD used for UPDATE and DELETE,
  -- are automatically created and populated by PostgreSQL whenever
  -- a data-change function is called.
  --
  IF NEW.customer_num = 0 THEN
    SELECT nextval('customer_customer_num_seq') INTO NEW.customer_customer_num;
  ELSE
    IF NEW.customer_customer_num > 0 THEN
      PERFORM setval('customer_customer_num_seq', NEW.customer_customer_num);
    END IF;
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

ALTER FUNCTION public.customer_num_informix() OWNER TO hll_4gl_testing;

CREATE TRIGGER customer_num_serial
  BEFORE INSERT ON customer
    FOR EACH ROW EXECUTE customer_num_informix();

The problem is that I am getting a syntax error on the CREATE TRIGGER statement:

ERROR:  syntax error at or near "customer_num_informix"
LINE 3:     FOR EACH ROW EXECUTE customer_num_informix();

I do not see what the error is. What is wrong with the syntax I used?

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