This is the code from a Task Pane Add-in that sends a message to a Content Add-in via a CustomXMLPart

SendCustomXmlPartMessage.js

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 sendMessage() {
  Excel.run(function(context) { 
    var data = xml.replace("[who]", from_tp).replace("[data]", "This message is coming from the taskpane.");
    const customXmlPart = context.workbook.customXmlParts.add(data);
    customXmlPart.load();
    return context.sync().then();          
  });
}