This is a function from a Content Add-in looks for a specific customXMLPart in the document as a message from a TaskPane Add-in

ContentAddInReadCustomXmlPart.js

Office.initialize = function(reason) {
  // background thread checker
  window.setInterval(() => { checkForPart(); }, 1000);
}

const ns = "http://pfe.microsoft.com/excelpoc/1.0";
const xml = "<message xmlns='http://pfe.microsoft.com/excelpoc/1.0'>" +
              "<sentby>[who]</sentby>" +
              "<info>[data]</info>" +
            "</message>";
const from_tp = "TASKPANE ADD-IN";
function checkForPart() {
  Excel.run(function(context) { 
    /**@type {Excel.CustomXmlPartScopedCollection} */
    var customXmlParts = context.workbook.customXmlParts.getByNamespace(ns);
    customXmlParts.load();
    return context.sync().then(function () {
      if(customXmlParts.items.length > 0) {
        /**@type {OfficeExtension.ClientResult<string>} */
        var xmlData = customXmlParts.items[0].getXml();
        context.sync().then(function() {
          /**@type {DOMParser} */
          var parser = new window.DOMParser();
          /**@type {Document} */
          var xmlDoc = parser.parseFromString(xmlData.value, "text/xml");
          /**@type {Element} */
          var who = xmlDoc.getElementsByTagName("sentby")[0];
          /**@type {Element} */
          var data = xmlDoc.getElementsByTagName("info")[0];
          document.getElementById("message").innerText = who.innerHTML;
          if(who.innerHTML == from_tp) {
            // write tot he pane
            var dt = new Date();
            var currentTime = pad(dt.getHours(),2) + ":" + pad(dt.getMinutes(),2) + ":" + pad(dt.getSeconds(),2);
            // update a DIV on the page
            document.getElementById("message").innerHTML = "<p>Message sent on " + 
                                                          currentTime + 
                                                          " by " + who.innerHTML + 
                                                          " and the message is " + data.innerHTML;
            // now we delete the part
            customXmlParts.items[0].delete();
            return context.sync();
          }
        });
      }
    });
  });
}