How use input parameter as path to COPY in function?

I am missing something basic. How can I  use an input parameter as the
destination path in a COPY statement in a function. The following fails
with a syntax error at or near the parameter.

CREATE OR REPLACE FUNCTION  dvd.export_tables(IN export_path text)
RETURNS void
AS
$$
BEGIN
  copy dvd.genre to export_path
  with
  delimiter as E'\t'
  null as '';
END;
$$
  LANGUAGE 'plpgsql' VOLATILE