Unrecognized token 'project' when creating new issue

I'm attempting to create new issue via the new API, but I'm having trouble with the syntax it seems.

{
    "project": { "id":"77-0"},
    "summary":"test",
    "description":"test"
}

And in response I get:

400 - Bad request

""Unrecognized token 'project': was expecting ('true', 'false' or 'null') at [Source: (org.glassfish.jersey.message.internal.ReaderInterceptorExecutor$UnCloseableInputStream); line: 1, column: 9]""

As far as I can tell, it's the same syntax as this example https://www.jetbrains.com/help/youtrack/standalone/api-howto-create-issue.html#, only difference being I'm using jQuery/AJAX to send it.

Any idea what's wrong?