Custom positioning of a signature box

I have a PDF & in my PDF I already know where I want to place the signature box.

I'm using this to create an agreement using the POST rest endpoint (v5)

{

    "documentCreationInfo": {

        "fileInfos": [

            {

                "documentURL": {

                    "mimeType": "application/pdf",

                    "name": "....",

                    "url": "http://example.com/example.pdf"

                }

            }

        ],

        "name": "Instructions",

        "recipientSetInfos": [

            {

                "recipientSetMemberInfos": [

                    {

                        "email": "someuser@gmail.com",

                        "fax": ""

                    }

                ],

                "recipientSetRole": "SIGNER",

                "recipientSetName": "Bob"

            }

        ],

        "signatureType": "ESIGN",

        "signatureFlow": "SENDER_SIGNATURE_NOT_REQUIRED",

        "formFields": [{

              "locations": [

                {

                  "height": 60,

                  "left": 60,

                  "pageNumber": 1,

                  "top": 90,

                  "width": 100

                }

              ],

              "name": "signature_1",

              "required": true,

              "contentType": "SIGNATURE_BLOCK",

              "displayLabel": "Signature Bob",

              "inputType": "SIGNATURE"

        }]

    }

}

However Adobe Sign/Echosign isn't acknowledging formFields and I just see the standard signature box when I get the signing URL. What can I do to control the position of the Signature boxes, so that they are not at the end of the pagr but they are placed at the corresponding top and left I assign to them.