Create an Asana task with External Data

I am having trouble creating an asana task with external data. I can create a task fine, but i can't figure out what the cocktail is to be able to use the "external" data that is mentioned in the documentation.

$.ajax({
   url : global.task.url,
   type: "post",
   data: {
    assignee: "my-email@hidden.com",
    name: "Test task",
    notes: "this is a note",
    projects: 123123123123,
    workspace: global.workspace,
    external: {
     "id": "test",
     "data": "12345099"
    }
   },
   beforeSend: function (xhr) {
    xhr.setRequestHeader("Authorization", "Bearer " + "MY-TOKEN");
   }
 })

the above code results in a 500 bad request.

I have also tried sending external stringified, which results in the error "No matching route for request".

The code above works perfectly if i remove the "external" from data.

i will be the first to say i do not typically call APIs so i feel like i must be doing something silly. any help appreciated!