REST API - Can't set the IterationPath

I'm writing a custom plugin that uses the REST API to create a bug in TFS. I'm sending it the Title, ReproSteps, Severity, and a hyperlink. My problem is I also want to set the IterationPath to something other than the default. Anything other than the default IterationPath doesn't work. I've looked through all the REST API documentation and the forums and it looks like I'm doing it correctly.

This is how I'm setting up the data.

{

     "op": "add",
     "path": "/fields/System.Title",
     "value": "'.$input['title'].'"
    },
    {
     "op": "add",
     "path": "/fields/Schilling.VSTS.Bug.SQA",
     "value": "'.$input['sqa'].'"
    },
    {
     "op": "add",
     "path": "/fields/Microsoft.VSTS.TCM.ReproSteps",
     "value": "'.$input['description'].'"
    },
    {
     "op": "add",
     "path": "/relations/-",
     "value": {
      "rel": "Hyperlink",
      "url": "'.$input['testurl'].'"
     }
    },
    {
     "op": "add",
     "path": "/fields/Microsoft.VSTS.Common.Severity",
     "value": "'.$input['severity'].'"
    },
    {
     "op": "add",
     "path": "/fields/System.IterationPath",
     "value": "'.$input['path'].'"
    }

Any help will be greatly appreciated.