How to post a JSON type parameter?

[Sher]

Sample:
$response = Zttp::asFormParams()->post($url, [
'foo' => 'bar',
'baz' => 'qux',
]);

'[ 'foo' => 'bar', 'baz' => 'qux',]' is a array? How to post '{ 'foo' : 'bar', 'baz' : 'qux'}'?