JSON Payload Help

Hello, I'm having a hard time structuring my JSON payload to reflect what I want to appear in my package. The documents have already been merged with *MOST* data via another source, but I do need to add some information to one of the 3 documents being sent for signing. This means I cannot build a new package with a form, but need to use the existing PDFs and structure I'm uploading to esign live. In my opinion, the easiest way would be to use a text anchor, find the location I want to add the data, and then give it a value and have esign live write it in. I would assume this is possible, since the same thing is done using signatures like this.

"approvals": [
                      {
                        "fields": [
                          {
                            "type": "SIGNATURE",
                            "extract": false,
                            "extractAnchor": {
                              "text": "(Signature of Member or Authorized Signatory)",
                              "index": 0,
                              "width": 300,
                              "height": 40,
                              "anchorPoint": "TOPLEFT",
                              "characterIndex": 0,
                              "leftOffset": 0,
                              "topOffset": -40
                            },
                            "left": 0,
                            "subtype": "FULLNAME",
                            "top": 0
                          }
                        ],
                        "role": "member"
                      }
                    ]

But I can't seem to get it to work no matter what I try, and the documentation is so confusing I can't figure out what to do next. I've tried something like this to no avail.

"fields": [
                      {
                        "name": "Principal Amount",
                        "value": "1,000,000",
                        "extractAnchor": {
                          "text": "Principal Amount",
                          "index": 0,
                          "width": 200,
                          "height": 40,
                          "anchorPoint": "TOPLEFT",
                          "characterIndex": 0,
                          "leftOffset": 0,
                          "topOffset": -40
                        },
                        "left": 0,
                        "subtype": "CUSTOMFIELD",
                        "top": 0
                      }
                    ]

I also can't seem to find the documentation that breaks down what each key should represent. I would appreciate any help someone could offer.