传递文本字段变量

I want to be able to pass a car registration text field through to a quotation engine. The end URL would be as below.

https://www.thedomain.com/default.aspx?op=getaquote&linkfrom=OMRMOTOR&brand=test&carreg=<?php echo $_SESSION['carreg']; ?>&campaign=<?php echo $_SESSION['campaign']; ?>&clickid=<?php echo $_SESSION['clickid']; ?>

I have created the following form but it doesn't appear to be passing the input of the car reg field, the clickID and campaign are though.

<form method="post" action="https://www.thedomain.com/default.aspx?op=getaquote&linkfrom=OMRMOTOR&brand=test&carreg=<?php echo $_SESSION['carreg']; ?>&campaign=<?php echo $_SESSION['campaign']; ?>&clickid=<?php echo $_SESSION['clickid']; ?>">
<input class="entry" id="carreg" onfocus="if(this.value == 'Your Reg') { this.value = ''; }" value="Your Reg" />
<input type="submit" value="Submit">
</form>

The problem is that the carreg isn't know in the session when that <form-Statement with the URL is bein parsed, because carreg will be entered in the form. So you should submit your form to an "processor" which could read the input and the redirect accordingly using header("location:https//www.thedomain.com/...etc.

http://www.w3.org/TR/html401/interact/forms.html#successful-controls

Only fields that have a 'name' attribute will be submitted by the form.