Sorry, it would be under the Add Email Account section:
1) Goto membertools.sonic.net and select email
on the left hand side after logging in. (Or click this link https://members.sonic.net/email/mailboxes/
2) Select Manage Mailboxes
3) It'll open a page with your existing mailboxes and on the left side is a text box under New Mailbox Name:
Type in the name of the email. (For example: JohnDoe would give you JohnDoe@sonic.net
) Then click continue.
4) If the username is available it will take you to a password page where you must type in a desired password for the new mailbox. After typing the password you'd like hit continue. That email should then be live and you would be able to log into it at webmail.sonic.net