Access to ArcGIS Online (AGOL) requires that you submit an account request to Bill Winston (billwinston@wustl.edu) or Mollie Webb (molliewebb@wustl.edu). Please include your name, department or school, WU email address, phone number, and the reason you are asking for access. Requests will be processed within 2 business days and you will receive a response.
Once your account has been setup you will receive an invitation email with a link to the AGOL login page:

Click on the link to access the Enterprise Login page.
Enter "wustl" as your Organization...
and click on the option to sign in "Using your WU Account"...
Finally...login with your WUSTL Key credentials. 
and please remember to fill out your AGOL Profile page to include your first AND last name.