Edit in JSFiddle

const data = {
  "dcids": [
    "geoId/5508"
  ]
}
const request = new XMLHttpRequest();
const wisconsin8thDistrict = "geoId/5508"
request.open("POST", "https://api.datacommons.org/node/property-labels", false /* async */ );
request.send(JSON.stringify(data));

if (request.status == 200) {
  let payload = JSON.parse(request.responseText)["payload"];
  let result = JSON.parse(payload);
  console.log(result);
  document.write("<h2>In Labels</h2>")
  for (let i = 0; i < result[wisconsin8thDistrict]["inLabels"].length; i++) {
    let resultName = result[wisconsin8thDistrict]["inLabels"][i];
    document.write(resultName + "<br>");
  }
  document.write("<h2>Out Labels</h2>")
  for (let i = 0; i < result[wisconsin8thDistrict]["outLabels"].length; i++) {
    let resultName = result[wisconsin8thDistrict]["outLabels"][i];
    document.write(resultName + "<br>");
  }


} else {
  console.log("There was an error fetching data.");
}

<h1>
Property labels of Wisconsin's 8th Congressional District
</h1>