How to add CSRF token to a form?

[eluser]Lpeek[/eluser]
Hi all, just wondering how I would add a CSRF token to a form? I expected I'd see a form_csrf() function or something in the form helper but can't see anything like that.

I guess it's automatically added by using form_open() if I were to use the helper to create my forms but what about if I'm creating forms manually?

Thanks!