request a function with JSON parameters

I have defined a function with JSON parameters. How should I request it in pg_tileserv?
 I have confirmed that the function is working properly on other servers
eg:

CREATE OR REPLACE FUNCTION function_xyz_query(
	z integer,
	x integer,
	y integer,
	query_params json)
    RETURNS bytea
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE PARALLEL UNSAFE
AS $BODY$
...