Current university email addresses for incoming graduates are entered into the system by the School in which you are enrolled. The detail of that process depend upon your school's enrollment office.
Some students are allowed to change their local email address in WebSTAC while others are not — this also varies by school and division. Contact your school's computing facilities for specific information.
To find your university email address, search our online directory. If the directory does not show an "@wustl.edu" address for you, then contact your school's computing facilities.