Pager Duty External reference

Hi, I’m trying to set an external referrence on a Pager Duty incident, in much the same way it’s done for the ServiceNow integration. However, whenever I submit a payload, I just receive something like this:

{
    "incident": {
        "errors": [
            "Invalid Input Provided"
        ],
        "id": "******",
        "type": "incident_reference",
        "summary": "[#437598] awr fgwg arewg ",
        "self": "https://api.pagerduty.com/incidents/******",
        "html_url": "https://node4.pagerduty.com/incidents/******"
    },
    "error": {
        "errors": [
            "Invalid Input Provided"
        ],
        "code": 2001,
        "message": "Invalid Input Provided"
    }
}

I cannot find documentation on this, but I’m trying to do as Doug describes in the following post: https://community.pagerduty.com/forum/t/including-links-in-rest-triggered-incidents/2209/8