If you pass the url parameter into the page where you have the plugin it will be available automatically. However if you want to pass a url parameter to the plugin page specifically you can set it directly in the code above with code like this:
<?php
$_GET['HomepageHeaderID'] = 1;
?>
Then if you have the plugin referenced with "homepage_header.php" it will work as if it was: homepage_header.php?HomepageHeaderID=1