I think the only way you can do it is like this:

<form method=post action=/gethostname>

    <label for=hostname>What is your hostname?</label>

    <input type=text name=hostname id=hostname>

</form>

But I would suggest you don't really need it, it's not like you can do anything useful with the information. If you just want a string to identify with the user's machine then you can make something up.

If what you're really after is the FQDN then I would suggest it's still not really that useful to you, but for that you need Reverse DNS lookup. If you're on a VPS or similar you can probably configure your box to do this for you, but note that it'll likely take a few seconds so it's not a good idea to do it as part of a response. Also note, you'll not be getting the user's machine's FQDN in most cases but that of their router.