Security :: 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:

<asp:TextBox ID="UserName" runat="server"></asp:TextBox>

Code-Behind:

protected void CreateUserWizard_CreatingUser(object sender, EventArgs e)
{ TextBox userNameTextBox = (TextBox)CreateUserWizardStep1.ContentTemplateContainer.FindControl("userName");
MembershipUser user = Membership.GetUser("AA" + userNameTextBox.Text); }