搜索表单将“sku”添加到网址

I have a search form that refreshes the page when the submit button is hit with the table of results below. this works fine and the url is http://example.com/?txtKeyword=searchterm I want it to add &sku=123456789 to the end of the url I know to add the sku from my database I need to use this term $_GET[sku]. I have tried to add this to the form and I have been unsuccessful. Here is the form code:

<form name="frmSearch" method="get" action="<?=$_SERVER['SCRIPT_NAME']?>">
  <table width="100%">
    <tr>
      <th>Search: (Keyword/SKU)
        <input name="txtKeyword" type="text" id="txtKeyword" value="<?=$_GET["txtKeyword"];?>" size="40">
        <input class="alert button" type="submit" value="Search"></th>
    </tr>
  </table>
</form>

I think I need to add this line:

<form name="frmSearch" method="get" action="<?=$_SERVER['SCRIPT_NAME']'&sku='$_GET["sku"]?>">

This does not work. I am not too sure, trying to learn PHP and I cannot seem to find the problem or a solution. Am I doing this wrong and what would I need to do to get this right?

It Depends if your script is looking for a $_POST['sku'] or $_GET['sku'] when you process it.

If it is a POST then you add a new hidden <input>

<input type="hidden" name="sku" value="XXXXX">

adding it to the form action will only work if it looks for $_GET['sku']

EDIT

If you wish to echo something if its there but not if it isn't then do something like this.

<?php echo !empty($_GET['sku']) ? '<input type="hidden" name="sku" value="'.$_GET['sku'].'">' : ""; ?>