Qusetion about using info user types into a textbox.

Hey Guys I am trying to use the email my user types into a text box to send them an email I can't figure it out though. everything else works if I manually put in the email address in the code it sends the email so I figured I would see if you guys could lend me a hand. so if you look in the code in the first function you will see I put auto fill with user typed email address and the ID for the text box they are typing it in is UserName. So I am trying to figure out what I put in place of auto fill to actually use the email address the user types in for username.
Thanks Guys,
Anthony
 
Here is the relevant code. 
<%@ Page Language="C#" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <%@ Import Namespace="System.Net.Mail" %> <script runat="server" ...