HOWTO pass "default value" to PQexecParams ?

-- SQL Script

Create Table test (
id int4 not null,
name text default '<noname>'
);

-- C

PQexec (conn, "INSERT INTO test(id, name) VALUES (100,default)" ); // OK.

PQexecParams (conn, "INSERT INTO test(id, name) VALUES ($1, $2)" ); //
$1=100, $2= ????