JSRuntime to get it working

commands.js

/**
 * Checks to see if we are running in Windows Outlook
 * @returns {Boolean}
 */
function isPC() {
  try {
    if (Office.context.platform === Office.PlatformType.PC || Office.context.platform === null) {
      return true;
    } else {
      return false;
    }
  } catch {
    return false;
  }
}
if (isPC() === true) {
  Office.actions.associate("onMessageSendHandler", onMessageSendHandler);
} else {
  Office.onReady(function () {});
  // Everything below is for OTHER (non-PC) clients per older constructs. Not certain
  // if any of this is needed except for COMMANDS (rather than EVENTS), so eventually
  // if commands are added, we have this in place...
  var g = getGlobal();
  // The add-in command functions need to be available in global scope
  g.onMessageSendHandler = onMessageSendHandler; // on send event
}
/**
 * OnSend event triggered
 * @param {Office.AddinCommands.Event} event
 */
function onMessageSendHandler(event) {
  Office.onReady(function () {});
  Office.context.mailbox.item.body.getAsync(Office.CoercionType.Text, { asyncContext: event }, function (asyncResult) {
    var body = asyncResult.value;
    var event = asyncResult.asyncContext;
    // ... more code here ...
 });
}