Passing UserName to JavaScript from CreateUserWizard.... I can't figure this out!!! :(

Hi Experts, I'm hoping someone can help me figure out how to reference the contents of a user name textbox on my register.aspx form so I can do some additional validation on it.

 Here's the control definition on my aspx page:

<form id="form1" runat="server">
        <div>
            <table width="100%">
                <tr>
                    <td style="width: 20%; height: 21px;">
                    </td>
                 ...