If you run the sample [Outlook-Event-SSO-NAA] in Outlook Classic, then it will popup an information bar to setup the signature.
However this doesn't poup in the web-version.
I added an extra 'commands.js' to the commands.html containing the next code:
// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
// This file contains code only used by autorunweb.html when loaded in Outlook on the web.
Office.initialize = function (reason) {};
and reference to it in the html, and then it does work ok.