如何使用Expression Web 4从asp中的DropDownList中检索选定的值？

I want to do some changes on existing website, working in Microsoft expression Web4. I want to email the chosen value of the dropdownlist , but i can't reach the selected value.

My code below :

<form id="form1" runat="server" action="send.php" method="post">

&nbsp;
<textarea cols="20" name="TextArea1" rows="2">
</textarea> 
<input name="Submit1" type="submit" value="submit" /><br />
<br />
<br />


<asp:DropDownList id="DropDownList5" runat="server" DataSourceID="SqlDataSource1" DataTextField="NAAM" DataValueField="NR" >
</asp:DropDownList>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="SELECT [NR], [NAAM] FROM [LID]">
</asp:SqlDataSource>

Now I want the selected value from the dropdownlist to php but I can't give my dropdownlist a name?

<?php 
 $to='*****@gmail.com';
 $subject='Inschrijving KRVGB';

 if(isset($_POST['Submit1']) )
 {

    $msg=$_POST['WhatDoITypeHere?'];
    mail($to,$subject,$msg);

 }

Can someone help or explain me what to do? I'm looking for hours..