Proper syntax to add a liquid snippet?

[phpcontrols]

I tried adding a snippet like this using Asset API

$snippet = ["asset['key']" => "snippets/newsnippet.liquid", "value" => "hello"];
$shopify->Theme($themeId)->Asset->put($snippet);

It doesn't seem to work. Returns 406.

I think the syntax have some issues but I can't figure out the correct format.