Here are the needed steps to sign into the Outlook Web Application Webex Plugin
1> Sign into Office365
2> Open the calendar tab, click New Event, and click on the webex icon
3> Select Add Webex Meeting
You will prompted to to Open another window in some browsers, please click allow which will open an Office 365 sign-in prompt
4> A prompt should appear to begin, you will want to click get started
5> If prompted for your webex site please enter newpaltz.webex.com
6> Then sign in using your Office 365 credentials
You may be prompted to sign in several times which is normal. This setup is only needed the first time you use the webex add-in.