substituting ETSubscriberKey in Web Collect

Question

I'm using ExactTarget's web collect functionality to collect new sign ups from our website.

We use custom susbscriber key as a unique identifier for a customer rather than a email address. our subscriber key would be something like client_code + Email address. Is there a way I concatenate the 2 inputs and put it as a value in ETSubscriberkey?

here is my code

       <form action="http://cl.s4.exct.net/subscribe.aspx" name="subscribeForm" method="post">
    <input type="hidden" id="thx" name="thx" value="">
    <input type="hidden" name="err" value="YOUR ERROR PAGE HERE" />
    <input type="hidden" name="usub" value="YOUR SUCCESS UNSUB PAGE HERE" />
    <input type="hidden" name="MID" value="1069927" />
    <INPUT TYPE="HIDDEN" name="ETSubscriberKey" VALUE="clientcode + emailaddress" />
    <INPUT TYPE="HIDDEN" name="Email Address" VALUE="[email protected]" />
    <INPUT TYPE="HIDDEN" name="CLIENT_CODE" VALUE="9999" />
    <INPUT TYPE="HIDDEN" name="lid" VALUE="32347" />
    <table cellpadding="0" cellspacing="0" border="0">
    <tr> 
<td><font face="Verdana" size="2">Email:</font></td> 
<td><input type="text" name="Email Address" id="EmailAddress"></td> 
</tr> 
        <tr>
            <td><input type="radio" name="Email Type" value="HTML" checked="checked" />
            <font face="Verdana" size="2">HTML</font>
        </td>
            <td>
                <input type="radio" name="Email Type" value="TEXT" />
                <font face="Verdana" size="2"> Text</font>
            </td>
        </tr>
        <tr>
            <td>
                <input type="radio" name="SubAction" value="add" checked="checked" />
                <font face="Verdana" size="2">Subscribe</font>
            </td>
            <td>
                <input type="radio" name="SubAction" value="unsub" />
                <font face="Verdana" size="2">Unsubscribe</font>
            </td>
        </tr>
        <tr>
            <td>
                <input type="submit" value="Submit" /></td>
                <td align="right">
                <font face="verdana" size="1" color="#4C5487">
                Powered by<br> <a href="http://www.exacttarget.com">
                <img src="http://www.exacttarget.com/gfx/smalllogo.gif" border="0"></a>
                </font>
            </td>
        </tr>
    </table>       

Answer 1

This worked

var subskey=document.getElementById('CLIENT_CODE').value +'-'+document.getElementById('EmailAddress').value;
document.getElementById('ETSubscriberKey').value = subskey;

Answer 2

You would want to use some kind of client side mechanism to do the concat upon submit. JQuery is probably the most popular flavor of client side manipulation in these parts.