Need to pass a variable in a search box

I have a variable in the url of ?refid=



I need to pass the value of this in a search box.
<form method="get" action="/search/">
<input type="text" value="" class="text" size="16" name="search">
<input type="submit" value="Go" class="searchbutton">
</form>

How do I grab the variable out and insert it into the form before it is submitted?

 

<input type="text" name="my_field" value="<?php echo $_GET['refid'];?>" />

Code (PHP):

 

Tweaked it a bit and got it working - thanks again for your help.

<form method="get" action="/search/">
<input type="text" value="" class="text" size="16" name="search">
<input type="hidden" name="ref" value="<?php echo $_GET['ref'];?>" />
<input type="submit" value="Go" class="searchbutton">
</form>