Is there a reason you want to log out from the form itself?

Keep in mind that this would redirect them so they wouldn't be able to submit the form unless you open it in a new tab or a frame, but that may cause issues since the form was opened with a logged in user.

The relative path for the logoff URL is /Forms/account/logoff

If you're looking log someone out after submission, then you can either set the form to redirect to the logoff URL instead of showing the "Thank You" page, or you could create a "log out" link on that page.