How do I append to a username during registration?

I want to append "AD" to the beginning of user names when they complete the CreateUserWizard.  I thought I could do it through the "oncreatinguser" event, but it won't add the "AD". Can someone help out with this:
ASPX:
Snippet
<asp:TextBox ID="UserName" runat="server"></asp:TextBox>
Code-Behind:
Snippet

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