Use $_POST
to get the id:
$format = "http://steamcommunity.com/profiles/%s?xml=1";
$url = sprintf($format, $_POST["SteamID64"]);
$xml=simplexml_load_file($url);
I'm assuming you've got the form set up correctly, something like:
<form action="TheTarget.php" method="post">
<input type="text" name="SteamID64" />
<input type='submit' value='submit' />
</form>