RESTful Webhook to Update Record

I've got a webhook coming from our customer survey application, that I want it to update a project in our project table with the results.  I have to use json because it doesn't like the xml webhook for some reason.  I set it up as a Insert/Update record.  My issue is that occasionally the results don't have a project ID (fid 3), and when the webhook sends, rather than just rejecting the update as an error, it creates a new project with the survey results.  How can I prevent the webhook from creating new records, but only update existing ones?

Here's what it looks like

{

  "to": "[DBID]",

  "mergeFieldID": "3",

  "data": [

    {

      "3": {

        "value": "${e://Field/QBProID}"

       },       

       "3513": {

        "value": "${rs://ResponseStatus}"

      }

}]
}

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Sean Boat-Moore
Manager SMB Development
Project Management Office and Quality Control
NCR Corporation
sean.boat-moore@ncr.com | ncr.com
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