oaktree8

I have a pretty basic form, it captures name and email and it stores the email in a input field dd_email. The form action is similar to below <form id="downloadform" name="form" class="cmxform" method="post" action="http://my.url.com/download-confirmation"> I want it to be like this but I can't get it to work: <form id="downloadform" name="form" class="cmxform" method="post" action="http://my.url.com/download-confirmation?utm_custom=$_POST['dd_mail']"> Is there a way to reference the email address submitted on the form as part of a UTM variable on the form action? If not how else can I do this so when it goes to the download-confirmation page it has that utm string? thanks