<script runat="server">
   Sub Check(sender As Object, e As EventArgs)
     if check1.Checked then
       work.Text=home.Text
     else
       work.Text=""
     end if
   End Sub
</script>

<!DOCTYPE html>
<html>
<body>

<form runat="server">
<p>Home Phone:
<asp:TextBox id="home" runat="server" />
<br>
Work Phone:
<asp:TextBox id="work" runat="server" />
<asp:CheckBox id="check1"
Text="Same as home phone" TextAlign="Right"
AutoPostBack="True" OnCheckedChanged="Check"
runat="server" />
</p>
</form>

</body>
</html>